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Performance requires that some opaque chunks (such as the background)
are drawn using a layer set to be opaque. This patch implements this
idea by adding a bit to DrawingDisplayItem for whether the item is
known to be opaque. The "knownToBeOpaque" bit is used to mark layers
as explicitly opaque which lets us pass 2x more tests.

BUG=580355

Review URL: https://codereview.chromium.org/1632263002

Cr-Commit-Position: refs/heads/master@{#371941}