[Chromoting] Add host support for SelectSource message from client.

This message is piped through to WebRtc's SelectSource to select
which display to show.

Bug: 915411
Change-Id: I30f10c8f94b15ebcff1c0fc0981385cbb8b7bc74
Reviewed-on: https://chromium-review.googlesource.com/c/1410229
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623544}
24 files changed