Merge 122791 - [chromium] Turn off ScrollbarLayerChromium for Windows due to bad alpha values

Merge 122791 - [chromium] Turn off ScrollbarLayerChromium for Windows due to bad alpha values
https://bugs.webkit.org/show_bug.cgi?id=91438

Reviewed by Kenneth Russell.

r120509 turned on blending for scrollbar thumbs. Unfortunately for
Windows XP scrollbar thumbs, their alpha channel is bogus and so the
thumb ends up being completely transparent. This should ultimately be
fixed in Chromium theme code, but in the meantime this patch turns
off the use of ScrollbarLayerChromium.

This change makes Windows scrollbars fall back to using normal
ContentLayerChromiums and being painted all in one layer on the main
thread rather than being painted separately and composited on the
compositor thread.

* page/scrolling/chromium/ScrollingCoordinatorChromium.cpp:
(WebCore::createScrollbarLayer):

TBR=enne@google.com
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=122854