PCScan: Defer reporting traces until scanning is done

Tracing scopes can do random unpredictable work that can lead to
problems such as a deadlock caused by acquiring a mutex recursively. The
CL avoids it by calling tracing macros after scanning has finished.

Bug: 11297512
Change-Id: I3eb1271287aec1190150034f970132e966c3bca6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2769898


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864692}