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We paint the main frame background by blending FrameView::
BaseBackgroundColor() and the background-color in LayoutView's
style (which comes from the HTML or BODY's style), but previously
we used only the latter for composited layer backgroundColor for
main frame.

Use the actual main frame background color. This will also make
SPv1 and SPv2 produce the same backgroundColor property for
the root composited layer after we let SPv2 support backgroundColor
of composited layers.

Also set background color for scrolling contents layer.

Change-Id: Ifbc84cc851545ab81f9aad12333f13a3447705cc
Reviewed-on: https://chromium-review.googlesource.com/638990
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#499069}