[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
tree. This patch marks the project as experimental which will give us
test and perf coverage on the bots.

Sheriffs: This shifts time (e.g., we now run PaintArtifactCompositor
and no longer run the cc property tree builder) and will likely change
performance benchmarks. Because this is an initial trial, feel free to
roll this patch out.

Bug: 836884
Change-Id: Ia1073bdd0922503925c17161f052bdfcd94196b7
Reviewed-on: https://chromium-review.googlesource.com/c/1292555Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601429}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -127,6 +127,7 @@
     },
     {
       name: "BlinkGenPropertyTrees",
+      status: "experimental",
     },
     {
       name: "BlinkRuntimeCallStats",