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We prefer to paint background-attachment: fixed into the scrolling
contents layer if text quality is important (at the cost of repainting
on every scroll). This approach had a bug for the LayoutView if there
turned out to be no scrolling layer (e.g., in a non-composited iframe).
The counterscroll logic in FixedAttachmentPositioningArea should
only apply if actually painting into a scrolling layer. This patch
queries the background compositing state before applying the counter-
scroll. This is safe because it occurs during paint which is after the
compositing decisions have been made.

Bug: 835755
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ieebffb50579b28488ef802736dd1210b8b27b26d
Reviewed-on: https://chromium-review.googlesource.com/1036941Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555814}