Fix crash with rtc_event_log in video_loopback
Fix crash with rtc_event_log in video_loopback

Because RtcEventLog is created and stopped in different threads,
SequencedTaskChecker causes failure at the end of a test.

Bug: none
Change-Id: Ibaec3162eedebd180b101ec46a171efee5fe667e
Reviewed-on: https://webrtc-review.googlesource.com/5401
Reviewed-by: Erik Språng <3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <76788ac3efd229e9d156135d9ceb7fef65c45da4@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20078}
ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc