tracing: Make startup tracing timeout more robust

A couple of refactorings that aim to make startup tracing setup and
timeout more robust:
1) Move startup tracing timeout from TraceEventDataSource into
   PerfettoProducer, so that it also affects other data sources.
2) Support aborting startup tracing in TracingSamplerProfiler.
3) Resolve the startup tracing target buffer reservation on abort in
   TraceEventDataSource.
4) Plumb the timeout through PerfettoTracedProcess to ensure no other
   concurrent tracing session is active (e.g. system tracing).
5) Move setup of TraceLog from trace_startup.cc into
   TraceEventDataSource.

Bug: 1006753
Change-Id: Ic6afb4ccc71fa960eb4db86d39450bc7c4c39992
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130353
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755837}