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Currently, FrameSinkManager::UnregisterFrameSinkManagerClient removes
the entry associated with |frame_sink_id| from |frame_sink_source_map_|
if the |frame_sink_id| does not have any children. This would result in
a client not connected with a BeginFrameSource when it is unregistered
and re-registered with the same FrameSinkId.

This CL removes the lines that removes the entry from |frame_sink_source_map_|
if the |frame_sink_id| has no child in
FrameSinkManager::UnregisterFrameSinkManagerClient. It also adds a unit
test to verify that the client gets reconnected to the BeginFrameSource
after unregistering and re-registering.

Bug: 735805
Change-Id: I93ae860b1192e23aa116749d0bbcdb553ddc2246
Reviewed-on: https://chromium-review.googlesource.com/563705
Commit-Queue: Xingyu Zhang <staraz@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485172}