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

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

This partially reverts Blink r157644
(https://codereview.chromium.org/23641004), which can cause a frame
that needs to scroll on the main thread to incorrectly scroll on the
compositor thread, causing non-composited fixed position elements to
bounce around.

BUG=296823
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=160720

[ajuma, 2013-10-24 15:05:53]

PTAL.

The problem is that ScrollingCoordinator is depending on whether the FrameView
is scrollable, but isn't notified when a FrameView's scrollability changes (e.g.
when its contentsSize() changes so that it's larger than the visible rect).
Since this needs to be merged to M31, the safest course of action is to revert.

[Ian Vollick, 2013-10-28 14:38:14]

On 2013/10/24 15:05:53, ajuma wrote:
> PTAL.
> 
> The problem is that ScrollingCoordinator is depending on whether the FrameView
> is scrollable, but isn't notified when a FrameView's scrollability changes
(e.g.
> when its contentsSize() changes so that it's larger than the visible rect).
> Since this needs to be merged to M31, the safest course of action is to
revert.

lgtm.

[commit-bot: I haz the power, 2013-10-28 14:38:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ajuma@chromium.org/40373002/1

[commit-bot: I haz the power, 2013-10-28 14:44:14]

Change committed as 160720