[BlinkGenPropertyTrees] Promote BGPT to experimental

BlinkGenPropertyTrees (BGPT) is an incremental step towards making
compositing decisions after the paint lifecycle phase. The primary
changes are to build property trees in blink and send a layer list to
the compositor, rather than building property trees in cc from a layer

This shifts time (e.g., we now run PaintArtifactCompositor and no
longer run the cc property tree builder) and will likely change
performance benchmarks.

Bug: 836884
Change-Id: I82ef1260baf5e71d9b44f0387bebbc218725b67c
Reviewed-on: https://chromium-review.googlesource.com/c/1363207
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614470}
164 files changed