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Since property tree builder makes assumptions about scrolling things
being snapped elsewhere, just have it snap scrollable things itself.
Use the Layer only to snap TextureLayers.

R=enne@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ia47bcaa252c6bdf58b276a7ff0bcde8de0af594e
Reviewed-on: https://chromium-review.googlesource.com/1100005Reviewed-by: enne <enne@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567406}