browser(webkit): fix scrolling with mobile viewport

[JoelEinbinder]

Previously setDelegatesScrolling was being called when we were in fixed layout mode on linux, but nothing was actually delegating the scrolling anywhere. This caused scrollIntoView to fail.

This patch does not fix scrolling with a mouse wheel when in headful mode + mobile viewport. Our mobile viewport causes scrollbars to be disabled, which also disabled mouse wheel scrolling.

#2179

[yury-s]

One thing that worries me is that after clicking on "About" link from the scenario in the bug report the new page stays blank for me. I've tried to click one of the links programmatically on that page but it didn't work

  const page = await context.newPage();
  await page.goto('https://www.google.com/?hl=en');
  await page.click('text="About"');
  await new Promise(r => setTimeout(r, 1000));
  await page.waitForLoadState();
  const link = await page.$('text=See the latest');
  console.log('found link: ' + link);
  await link.click('text=See the latest');

In any case there are probably more rendering issues in fixed layout mode and GTK, we can look at them separately.