Tracing: Flag event emitted when no PostTask is possible

Perfetto can end up PostTasking in certain situations (like when
creating a TraceWriter, or wanting to commit a chunk of data);
while the taskqueue is locked this isn't possible so we need to
flag these events to enable Perfetto to avoid PostTasking
under these circumstances.

R=gab@chromium.org

Bug: 898597
Change-Id: I3e1e35ec9858203dbdc8ac8560cdedda45aafb6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531611
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642668}
3 files changed