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Setting opacity on a layer with children that draws leads to using a
render pass, which is super expensive. Use the usual trick of setting
opacity on the leaf layers directly instead.

Change-Id: If394f0bba0953b9c6b951d2f320b358fe16c8acb
Reviewed-on: https://chromium-review.googlesource.com/1238566Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593556}