Merge 92030 - [chromium] Only force slow scrolling for iframes under --force-compositing-mode

Merge 92030 - [chromium] Only force slow scrolling for iframes under --force-compositing-mode
https://bugs.webkit.org/show_bug.cgi?id=65394

Reviewed by James Robinson.

The previous code introduced by r91591 made composited frames as well
as non-composited subframes go down the slow scrolling path.  Adjust
the check to use ownerElement to test for subframe and use
hasCompositedContent() which is a more correct test for compositing
than isEnclosedInCompositingLayer.

* page/FrameView.cpp:
(WebCore::FrameView::useSlowRepaints):


TBR=enne@google.com
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=92299