Derive draw transforms from property trees.

Derive draw transforms from property trees.

This patch computes draw transforms (aka target space
transforms) in terms of the property trees and verifies
that the match those computed by CDP.

This required implementing scroll snapping and
refactoring transform updates so that scroll offset and
transform updates can easily be updated (and snapped)
without a full tree build.

BUG=386810

Review URL: https://codereview.chromium.org/886813003

Cr-Commit-Position: refs/heads/master@{#315466}