Enable hardware-accelerated video decoding for GPU tests
Enable hardware-accelerated video decoding for GPU tests

This is a change to the GN config to sync with changes to the flags passed in
https://codereview.chromium.org/1488653003/.

R=dpranke@chromium.org, kbr@chromium.org, phajdan.jr@chromium.org
BUG=563847

Review URL: https://codereview.chromium.org/1503463003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363887}

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