fix: Update existing namespaces in api-metadata.json

[Rob--W]

I have auto-generated the schema signatures for Chrome 54 and Chrome 67, and manually merged the result, as follows:

• Ensure that the keys are alphabetically sorted.
• Remove keys that are not implemented by either browser.
• Add methods that are supported by both Firefox and Chrome.
• Add fallbackToNoCallback where needed.

This PR only modifies existing namespaces. Missing namespaces will be added by other PRs.

Fixes #26
Fixes #93

The diff between the output of #122 and the api-metadata after this is as follows (I added comments to explain the discrepancies):

164,178d163
  The tool from #122 does not generate the devtools namespace
<   "devtools": {
<     "inspectedWindow": {
<       "eval": {
<         "minArgs": 1,
<         "maxArgs": 2
<       }
<     },
<     "panels": {
<       "create": {
<         "minArgs": 3,
<         "maxArgs": 3,
<         "singleCallbackArg": true
<       }
<     }
<   },
179a165,168
  Firefox does not support acceptDanger.
>     "acceptDanger": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
196,200d184
  Chrome's downloads.open method does not take a callback.
  (and the tool omits all non-callback methods).
<     "open": {
<       "minArgs": 1,
<       "maxArgs": 1,
<       "fallbackToNoCallback": true
<     },
216,220d199
  Chrome's downloads.show method does not take a callback.
  (and the tool omits all non-callback methods).
<     },
<     "show": {
<       "minArgs": 1,
<       "maxArgs": 1,
<       "fallbackToNoCallback": true
230a210,221
  Firefox does not support extension messaging in the extension namespace.
  (only in the runtime namespace)
>     },
>     "sendMessage": {
>       "minArgs": 1,
>       "maxArgs": 3
>     },
>     "sendNativeMessage": {
>       "minArgs": 2,
>       "maxArgs": 2
>     },
>     "sendRequest": {
>       "minArgs": 1,
>       "maxArgs": 2
269a261,272
  Firefox's identity API does not support several of Chrome's.
  https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/identity#Functions
  https://developer.chrome.com/extensions/identity#toc
>     "getAccounts": {
>       "minArgs": 0,
>       "maxArgs": 0
>     },
>     "getAuthToken": {
>       "minArgs": 0,
>       "maxArgs": 1
>     },
>     "getProfileUserInfo": {
>       "minArgs": 0,
>       "maxArgs": 0
>     },
272a276,279
>     },
>     "removeCachedAuthToken": {
>       "minArgs": 1,
>       "maxArgs": 1
281a289,296
  Firefox's management API does not support all of Chrome's APIs
  https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/management#Functions
  https://developer.chrome.com/extensions/management#toc
>     "createAppShortcut": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
>     "generateAppForLink": {
>       "minArgs": 2,
>       "maxArgs": 2
>     },
289a305,312
>     "getPermissionWarningsById": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
>     "getPermissionWarningsByManifest": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
293a317,320
>     "launchApp": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
297a325,332
>     "setLaunchType": {
>       "minArgs": 2,
>       "maxArgs": 2
>     },
>     "uninstall": {
>       "minArgs": 1,
>       "maxArgs": 2
>     },
364c399
  Chrome does not support runtime.getBrowserInfo,
<     "getBrowserInfo": {
---
  Firefox does not support runtime.getPackageDirectoryEntry
>     "getPackageDirectoryEntry": {
379a415,418
  Firefox does not support runtime.restartAfterDelay
>     "restartAfterDelay": {
>       "minArgs": 1,
>       "maxArgs": 1
>     },
416a456,459
  Firefox does not define storage.management.clear/remove/set.
  Chrome does, because it re-uses the same class for all storage APIs.
>       "clear": {
>         "minArgs": 0,
>         "maxArgs": 0
>       },
423a467,474
>       },
>       "remove": {
>         "minArgs": 1,
>         "maxArgs": 1
>       },
>       "set": {
>         "minArgs": 1,
>         "maxArgs": 1
477a529,532
  Firefox does not support tabs.getAllInWindow
  (MDN currently says that it does, but that is wrong and I will submit a fix for that)
  Chrome has deprecated it in favor of tabs.query.
>     "getAllInWindow": {
>       "minArgs": 0,
>       "maxArgs": 1
>     },
481a537,540
  Firefox does not support tabs.getSelected
  Chrome has deprecated it in favor of tabs.query.
>     "getSelected": {
>       "minArgs": 0,
>       "maxArgs": 1
>     },
514,517d572
  Chrome does currently not support tabs.removeCSS
  It may be added in the future - https://crbug.com/608854
<     "removeCSS": {
<       "minArgs": 1,
<       "maxArgs": 2
<     },
521a577,580
  Firefox does not support tabs.sendRequest (which is deprecated by Chrome)
>     "sendRequest": {
>       "minArgs": 2,
>       "maxArgs": 2
>     },