[RLS] Remove PaintLayerCompositor layers

This patch removes the !RLS codepaths in PaintLayerCompositor (PLC)
because PLC does not create layers with Root Layer Scrolling (RLS). Now
RLS has launched, we can remove this obsolete technology.

Bug: 823365
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I999a7239c88acd48ba781ff5bcdcd6079994daaa
Reviewed-on: https://chromium-review.googlesource.com/1072487
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561678}
8 files changed