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This CL makes FtlMessagingClient ignore broadcasted message that doesn't
have a sender registration ID. This can happen when the user registers
a new device, which will broadcast a USERDATA message to all of the
user's devices. If this happens with a CHROMOTING_MESSAGE then it means
the sender has bug.

Currently we still ack messages with no sender registration ID, since
normally we don't expect Chromoting hosts/clients to broadcast. And if
someone (might be FTL backend) does broadcast, we want to make sure the
message won't just sit in the inbox forever.

Change-Id: Ib99796675918aae8ddcaf572c032d3fb8b929771
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626653
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662488}