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This ensures that the actual WebContents is used to track desktop
capture in MediaCaptureIndicator. Prior to this CL a null WebContents
was used.

Bug: 1140487
Change-Id: I3078dc3d071ca1dc34716de491996a275026f793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488160Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819335}