Use CrossThreadWeakPersistent for PeerConnectionTracker in CreateSessionDescriptionRequest.

CreateSessionDescriptionRequest is used cross-thread and accordingly
should use a CrossThreadWeakPersistent to reference the
PeerConnectionTracker. The CL also re-enabled a test that was flaky
because of this (disabled by the sheriff).

Bug: 1183761,1183974
Change-Id: Ibbbef4b7b54b9415899b2e233c7ca2200048860f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2730373
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Tal Pressman <talp@google.com>
Cr-Commit-Position: refs/heads/master@{#859292}