Add a missing lock in VideoBroadcaster::OnDiscardedFrame().

VideoBroadcaster is marked as thread-safe, but that is currently not the
case as OnDiscardedFrame() iterates through an std::vector of sinks in
VideoSourceBase that is not thread-safe and elements of that std::vector
are added/removed in AddOrUpdateSink()/RemoveSink() that could be called
on a different thread.

Bug: None
Change-Id: I5b61127f7ea6ce7f1322c5e770ab56643d7bd0d3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208404
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Mirta Dvornicic <mirtad@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33313}