cc: More documentation of the Layer API

And SetShouldFlattenTransformFromPropertyTree() becomes
SetShouldFlattenScreenSpaceTransformFromPropertyTree() as per the TODO
to rename it more appropritely.

R=enne@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I783e8474f39899de99ca6cf51e102f5ea604b836
Reviewed-on: https://chromium-review.googlesource.com/1101915
Reviewed-by: weiliangc <weiliangc@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568235}

• cc/layers/layer.cc[diff]
• cc/layers/layer.h[diff]
• cc/layers/layer_impl.cc[diff]
• cc/layers/layer_impl.h[diff]
• cc/trees/draw_property_utils.cc[diff]
• cc/trees/property_tree_builder.cc[diff]