PCScan: Fix races on insertion to DeferredTraceEventMap

Multiple mutators can insert into the same map. Make sure it's thread
safe. Scanner's map is only used from a single thread, but keep it
consistent with the mutator's one.

Bug: 11297512
Change-Id: Iead50e98fc8032f0b4412c9a92ca69f6985a5ecb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2775488


Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864954}