[CompositeAfterPaint] More accurate compositing reasons in layer tree dump

The CAP compositing algorithm may mark many layers with direct compositing
reasons as kOverlap, but we should try to get the actual reason. This is
just for testing and debugging.

Change-Id: I9a6b707215ed02232ac001a0485987efbca7e2d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227261
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775089}