[tsan] Guard TestStatictics against concurrent access.
[tsan] Guard TestStatictics against concurrent access.

This CL quiets ThreadSanitizer for EventTracerTest.ScopedTraceEvent unit test.

Bug: webrtc:10943
Change-Id: Ifaec1ab37b85f091872f55e630189387f8df4401
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151309
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29090}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc