Add target.evaluate and target.evaluateHandle

[ebidel]

Related to #1215.

Lighthouse has started testing its chrome extension using pptr. They currently usecreateCDPSession to inject code into the extension and run methods:

    const targets = await browser.targets();
    const extensionTarget = targets.find(({_targetInfo}) => {
      return _targetInfo.title === 'Lighthouse' && _targetInfo.type === 'background_page';
    });

    if (!extensionTarget) {
      return await browser.close();
    }

    const client = await extensionTarget.createCDPSession();
    const lighthouseResult = await client.send('Runtime.evaluate', {
      expression: `runLighthouseInExtension({
          restoreCleanState: true,
        }, ${JSON.stringify(lighthouseCategories)})`,
      awaitPromise: true,
      returnByValue: true,
    });

It would be nice to make this use case easier:

const lighthouseResult = target.evaluate(cats => {
  return runLighthouseInExtension({restoreCleanState: true}, cats);
}, lighthouseCategories);

[aslushnikov]

It should be possible to create a Page out of extension's background page. So we should simply teach target.page() to work with background_page.