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Observe the target WebContents for Tab/OffscreenTab mirroring.

Bug: 734672
Change-Id: I65194a345f81b38967396c80e5f614d4119782e4
Reviewed-on: https://chromium-review.googlesource.com/1176110Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586097}