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Per crbug/444930 discussion, BrowserCdm should generally remain on UI
thread. Interactions between the CMA pipeline and BrowserCdm should
remain thread-safe.

R=erickung@chromium.org,lcwu@chromium.org

Review URL: https://codereview.chromium.org/903083003

Cr-Commit-Position: refs/heads/master@{#315359}