GitLab

This CL is changing the kind of observer used by the sampling profiler
wrapper class. The proposed implementation is to use the sync version
which is fixing early startup tracing/sampling.

The previous code was using a Async observer to keep the class
thread-safe. Unfortunately, the sampling profiler is not able to start
early enough on startup. The hooks are at the right place.

The reason why it's not working on startup is because the tasks queue is
already loaded of startup tasks to be executed on the main thread.
Thus, after activating tracing, there is a long gab between the point
where the tigger occurs and when the sampling profiler got created and
start to collect samples (e.g. OnTraceLogEnabled).

To avoid this delay, we prefer using a Sync observer. That forces the class
to be thread-safe.

R=oysteine@chromium.org
CC=​ssid@chromium.org, wittman@chromium.org

Change-Id: I02a1deb2f8aa705ccbf5fb6e7026f4588ca1b319
Reviewed-on: https://chromium-review.googlesource.com/c/1359293Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614784}