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This CL keeps OffscreenTabOwner in TabCapture API, but moves
OffscreenTab to chrome/browser/media to allow it being used by the
mirroring service to support OffscreenTab mirroring. Implementation is
not changed at all. No functional or behavior change either.

Bug: 734672
Change-Id: Ia8de3680853b86824809d70dc96c6180e9a0c957
Reviewed-on: https://chromium-review.googlesource.com/1180588
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586101}