[CompositeAfterPaint] Fix paint/invalidation/background/full-viewport-repaint-for-background-attachment-fixed.html

Whether to invalidate LayoutView's fixed-attachment background should
only depend on whether we are painting the background onto the scrolling
contents layer.

Bug: 644358
Change-Id: Idbe415235d2fc911e90cded978928a37b5f4835a
Reviewed-on: https://chromium-review.googlesource.com/c/1418118
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623868}
5 files changed