Paint local attachment backgrounds into composited scrolling contents layer
This change detects when we can paint the background onto the scrolling contents layer. This will unblock promoting scrollers with supported backgrounds on low DPI devices as we can now maintain subpixel antialiased text.
BUG=381840,568847
TEST=compositing/overflow/overflow-scroll-with-local-image-background.html, LayoutBoxTest.BackgroundRect, CompositedLayerMappingTest.ShouldPaintBackgroundOntoScrollingContentsLayer
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Review-Url: https://codereview.chromium.org/2068723002
Cr-Commit-Position: refs/heads/master@{#414795}
26 files changed
