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When RLS is enabled, the compositor's root content layer is pointless,
and it doesn't get its setMasksToBounds flag set based on whether it
contains the root scroller.

Instead, we should ensure that the LayoutView's scrolling layer gets
the setMasksToBounds flag set correctly.  This patch adds some spacers
to make the LayoutView overflow, to ensure that it has a scrolling
layer we can test.

BUG=490942
R=bokan@chromium.org,skobes@chromium.org

Review-Url: https://codereview.chromium.org/2576943002
Cr-Commit-Position: refs/heads/master@{#438727}