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This CL teaches WebMVideoClient to parse the |codec_private|
field, if present, to figure out the VP9 Profile if any. This
is a necessary step to support VP9.2 10b on Vaapi.

TEST=crosvideo.appspot.com/?codec=vp9.2&loop=true and newly
added unittests.

Bug: 778093, 592074
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;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
Change-Id: Ic62519b32818fe409d15f54b6ddb9798deea88da
Reviewed-on: https://chromium-review.googlesource.com/786211
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519519}