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With RLS, we don't paint anything into this layer unless we have a fixed
background.  Update the SetDrawsContent logic to reflect this.

Patchset 15 contains the core logic change along with unit test updates
and a fix to compositing/overflow/composited-scroll-overlap-test.html.

The diff from patchsets 15..16 updates 2685 layout test baselines to
include "drawsContent": false in the layer tree output, and was produced
by this script: https://pastebin.com/wtcsSnL0

The diff from patchsets 16..17 updates additional baselines missed by
the script.  These were made with webkit-patch rebaseline-cl, patched
per http://crbug.com/812784#c8, plus some manual fixups.  The following
additional differences are observed in these baselines:

- Some paint invalidations on the main GraphicsLayer are no longer
  recorded, because the layer does not draw content.

- Some tests stuff the layer tree JSON into their DOM, so the scrolling
  contents layer's "bounds" value reflects more lines of text.

- The devtools LayerTreeModel object dumps fewer layers because it
  determines contentRoot() based on drawsContent.

- A layout invalidation on the document has a different reason code in
  timeline-paint-with-layout-invalidations.js (not fully understood).

Bug: 811429
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_layout_ng;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: If6f4da9961c83cbf96454475ebb5727d90045238
Reviewed-on: https://chromium-review.googlesource.com/920798
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538702}