Ignore should-scroll-on-main-thread if main frame is not scrollable

Ignore should-scroll-on-main-thread if main frame is not scrollable

This change had been reverted previously because of LayoutTest flakiness. I had
been concerned that this was because the code was actually incorrect, but it
looks like the real problem is that we're not correctly resetting between
layout tests inside Internals::resetToConsistentState. I've updated that fn
and the tests are no longer flaky.

This reverts commit 757a502a1b21193db4b84db6cff0703c112291ef.
Review URL: https://codereview.chromium.org/76983003

BUG=321358
R=ojan@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=162663

[Ian Vollick, 2013-11-25 19:35:26]

Most of this is a straight reland of old code.

What's new:
 - ScrollingCoordinator::needsToUpdateAfterCompositingChange now returns true if
the FrameView's answer to isScrollable has changed.

  - Internals now resets the scrolling coordinator and the frame view.

Ojan, can you PTAL?

[Ian Vollick, 2013-11-25 21:58:52]

On 2013/11/25 19:35:26, Ian Vollick wrote:
> Most of this is a straight reland of old code.
> 
> What's new:
>  - ScrollingCoordinator::needsToUpdateAfterCompositingChange now returns true
if
> the FrameView's answer to isScrollable has changed.
> 
>   - Internals now resets the scrolling coordinator and the frame view.
> 
> Ojan, can you PTAL?

+jchaffraix, in case you've got a moment to look at this re-land.

[ojan, 2013-11-25 22:26:44]

lgtm

[commit-bot: I haz the power, 2013-11-25 22:27:01]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vollick@chromium.org/85763003/20001

[commit-bot: I haz the power, 2013-11-26 00:55:50]

Retried try job too often on win_blink_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_blink_...

[Ian Vollick, 2013-11-26 03:01:43]

Committed patchset #2 manually as r162663 (presubmit successful).