Add slim::LayerTreeImpl

Features added in this CL:
* Requesting and releasing FrameSink.
* Most tree properties such as visibility, viewport, scale,
  background color,
* Tracking various begin frame requests calling client BeginFrame.
* Defer begin frame.
* Various LayerTreeClient callbacks though most of these are not
  exercised yet.

Bug: 1408128
Change-Id: I1ff46431dc06608151c18c3f6f79688b3b8c61ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4219268
Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Commit-Queue: Bo Liu <boliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1103513}