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Running out of array levels can cause the VP9 MFT to hang waiting for
some picture buffers to be released. This shouldn't be able to happen
(pending_output_samples_ should have at most one element), but save
information so we can be sure.

BUG=636158, 613701
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2590723002
Cr-Commit-Position: refs/heads/master@{#439873}