Because IPC call runs asynchronously in both remote decoder process and
content process, ProcessOutput() for buffers prior to Flush() could be
scheduled to run after the flush promise is resolved, and Codec.queueInput()
could be preempted and processes prior sample after flush.
To help check the validness of buffers, a session ID increased by flush
is added to both RemoteDataDecoder and remote codec service and will be
passed through IPC. If the passed ID doesn't agree with current session
ID, it means the buffer doesn't belong to current session and should be
discard.
Differential Revision: https://phabricator.services.mozilla.com/D36382
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