AllocationSequence uses legacy rtc::Thread message handling. In order
to cancel callbacks it uses rtc::Thread::Clear() which uses locks and
necessitates looping through all currently queued (unbounded) messages
in the thread. In particular, these Clear calls are common during
negotiation and the probability of having a lot of queued messages is
high due to a long-running network thread function invoked on the
network thread.
Fix this by migrating AllocationSequence to task queues.
Bug: webrtc:12840, webrtc:9702
Change-Id: I42bbdb59fb2c88b50e866326ba15134dcc6ce691
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/221369
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34241}
da8a45fdaa