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This will be needed to move the "paused message queue" into the
ChannelEndpoint, since transferring messages from the
LocalMessagePipeEndpoint needs to happen "atomically" with its
conversion to a ProxyMessagePipeEndpoint. (Before the MessagePipe lock
is released, the ProxyMessagePipeEndpoint needs to have a place to queue
messages, and any already-queued messages must be at the front of that
queue.)

R=darin@chromium.org

Review URL: https://codereview.chromium.org/585093002

Cr-Commit-Position: refs/heads/master@{#295822}