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We are seeing half of FrameSequenceLength.Video reports length 0.
What's the problem? It turns out that those half are from the main
thread. A video frame is on impl thread only, so main thread gets
0 frames for a video tracker.

This CL fixes it.

Bug: 1112893
Change-Id: I0f6b41625ac2d781dc9c0599e0e8ad28c06a033d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342247Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795949}