[chromium] Mark root layer scrollbars as always opaque to disable blending

[chromium] Mark root layer scrollbars as always opaque to disable blending
https://bugs.webkit.org/show_bug.cgi?id=79951

Reviewed by James Robinson.

Source/WebCore:

Now that scrollbar layers exist, mark non-overlay root scrollbars as
opaque. This disables blending for correctness and performance.

* page/scrolling/chromium/ScrollingCoordinatorChromium.cpp:
(WebCore::scrollbarLayerDidChange):
(WebCore::ScrollingCoordinator::frameViewHorizontalScrollbarLayerDidChange):
(WebCore::ScrollingCoordinator::frameViewVerticalScrollbarLayerDidChange):
* platform/graphics/chromium/LayerRendererChromium.cpp:
(WebCore::LayerRendererChromium::drawTileQuad):

LayoutTests:

Mark canvas-text-alignment.html as failing on Linux in general, not
just in debug. I believe that with this change, the same image will
pass on both release and debug, but will land that change separately.

* platform/chromium/test_expectations.txt:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@110620 268f45cc-cd09-0410-ab3c-d52691b4dbfc