[RLS] Remove CompositedLayerMapping's fixed root background layer

CompositedLayerMapping's background_layer was only used for fixed root
backgrounds (see: PLC::NeedsFixedRootBackgroundLayer) because the main
graphics layer scrolled. With root layer scrolling, all fixed
backgrounds are painted on the main graphics layer, and we no longer
need CLM's background_layer.

Bug: 823365
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: I1546edb5fa1588b7499328b1ec56327e379f847b
Reviewed-on: https://chromium-review.googlesource.com/1067076
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560321}
17 files changed