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This calls it from OnLayerTransform(). That way, if the transform is
changed outside of Window::SetTransform, everything still works.

BUG=822917
TEST=covered by tests

Change-Id: I27acb266c4a4993df0a71457af8aa3b4c2cde5d1
Reviewed-on: https://chromium-review.googlesource.com/967841Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543974}