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In MojoVDService and some unittest we duplicate a simple
implementation of VideoFrame::SyncTokenClient. This CL moved it to a
separate file.

BUG=522298
TEST=media_unittests

Change-Id: I18009712977a12b31cc8136ef79a230b34667384
Reviewed-on: https://chromium-review.googlesource.com/c/1460741
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Auto-Submit: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630692}