Page#setViewport required options

[SheetJSDev]

	await page.setViewport({width: WIDTH});

with no height, puppeteer rejects with the error message:

Protocol error (Emulation.setDeviceMetricsOverride): Invalid parameters width: integer value expected

However, the documentation https://pptr.dev/#?product=Puppeteer&version=v1.10.0&show=api-pagesetviewportviewport does not indicate that both height and width are required.

IMHO setting just the width should change the width while leaving the height untouched (default being the current width/height). However, if the current behavior is desirable, the docs should be updated to reflect that. Other functions like page#setCookie explicitly mark fields as required in bold, and the same thing should probably happen here.

[aslushnikov]

However, if the current behavior is desirable, the docs should be updated to reflect that.

I'd appreciate a PR.

[Nazanin1369]

I'd love to take this