It was found that generator can be destroyed on another thread
comparing to the one, from which frame were generated. It can happen
because generator injected into PC though scoped_ref object and the
last pointer to that object can be destroyed on different thread
depending on machine load.
To fix this sequence checker is replaced with lock. It is required
to ensure that generator won't be destroyed while it is reading frame,
because otherwise it can catch SIGSEGV.
Bug: webrtc:10138
Change-Id: Ia3488bd8ae396c209b90977469593784bb82114b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/162182
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30086}
9d06bc2e6d