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We currently assume that the occlusion calculated for contributing
surface (the second last element of the stack in occlusion tracker) is
always the occlusion by its target. This is not true.

BUG=501135
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2147283002
Cr-Commit-Position: refs/heads/master@{#405611}