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Now that we use the PaintOpBuffer, analyze whether a DisplayItemList
has any discardable images to skip metadata generation if unnecessary.

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2830243002
Cr-Commit-Position: refs/heads/master@{#469509}