GitLab

There are many TraceLog observers that are just tracing clients which
add a feature to tracing. These features are expected to live as long
as tracing is possible. So, make TraceLog own such observers instead
of creating singletons everywhere.

BUG=859260

Change-Id: Iebdaf128f11373f19a27415b66b256ef9a2c72f3
Reviewed-on: https://chromium-review.googlesource.com/1195847Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Siddhartha S <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587369}