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Add a feature flag to enable VA-API-based VP8 hardware video encode
acceleration.

BUG=794608
TEST=run Chrome with feature disabled/enabled, verify availability

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I5866d7b158a071ea94b06d49453af1ec85ba3e2e
Reviewed-on: https://chromium-review.googlesource.com/1068905Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561303}