In the following example, we do not need to composite the first transform:
<div id="first" style="transform: translate(10px, 10px); width: 100px; height: 100px;">
  <div id="second" style="will-change: transform; width: 50px; height: 50px;"></div>
</div>

Pre-BlinkGenPropertyTrees, this was required to ensure a composited
cc::TransformNode existed, but this can now happen without a composited
cc::Layer.

This should reduce memory: On the top 10k pages, 2% of GPU tile memory is due to
layers with the kTransformWithCompositedDescendants compositing reason. This was
determined using a metrics collection patch on cluster telemetry:
https://docs.google.com/spreadsheets/d/1cCFRHDG9Uu0dbGETmHcjM-QI2nMZe8_lfA8Mgm7H23I/edit#gid=2004092469

Bug: 989251
Change-Id: I2b111eb2fe5cdbbfe073846f4e1271fb454bd887
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696112Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692043}