Composited fixed-pos element should still cause repaints if it paintsIntoCompositedAncestor

Composited fixed-pos element should still cause repaints if it paintsIntoCompositedAncestor

A composited fixed-position element may still actually be set to
paint into its composited ancestor, even though it has its own backing.
ScrollingCoordinator and FrameView were not recognizing this situation
and thus the fixed-position element did not repaint, appearing to be
attached to the scrolling contents rather than the viewport.

This patch makes the FrameView and ScrollingCoordinator aware of the
scenario so that main-thread scrolling is forced and repaints occur.

This patch is an exact re-land of 157920, which was reverted in 158540.
It was reverted only because it had a very badly misleading description.

TBR=enne@chromium.org
BUG=151734
NOTRY=true

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=158541

[commit-bot: I haz the power, 2013-09-30 18:11:25]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shawnsingh@chromium.org/25110005/1

[commit-bot: I haz the power, 2013-09-30 18:11:38]

Change committed as 158541