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|registered_frame_sink_ids_to_is_connected_| and |frame_sink_elements_|
saves the same information about the FrameSinkId, that is if a
FrameSinkId is register and/or created. This cl removes the variable
|registered_frame_sink_ids_to_is_connected_|.

Bug:816802

Change-Id: Iad9c9e9819ffd44ca8aff7c7b82ad0b5852d6e8d
Reviewed-on: https://chromium-review.googlesource.com/1186195Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585859}