Fix UiThreadGpu initialization for video accelerator tests

The UiThreadGpu shouldn't be initialized multiple times for the same
OzonePlatform. This fixes failing DCHECKs in the decode accelerator
unittests.

BUG=447798

Review URL: https://codereview.chromium.org/936703002

Cr-Commit-Position: refs/heads/master@{#317474}
9 files changed