This prevents a race where two threads try to write events
simultaneously.

Bug: b/172704839
Change-Id: Idd2f3b1c2a393488ea2270b91d579dc8630e59d5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2533496
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>