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This CL is making sure tracing categories are
activated as soon as the request for start tracing
is done. Background tracing (Slow-reports) is
sending a request to TraceLog which delegate it
to the tracing service. Starting tracing is not
synchronous. The categories are activated after
the tracing is started.

This CL is manually turning on the categories by
calling SetEnabled on TraceLog.

A manual trigger for session is added. This trigger
is reactive which means tracing won't be activated
on a normal usage of chrome. It will only be
activated during session restore (depending on the
background tracing configuration).


R=chrisha@chromium.org, oysteine@chromium.org

This CL can locally be tested with:

  out\build\chrome.exe --enable-background-tracing=c:\src\chromium\tracing-config.json --trace-upload-url=http://localhost:8080

==== trace-config.json ====
{
  "scenario_name": "Browser",
  "mode": "REACTIVE_TRACING_MODE",
  "configs": [
    {
      "rule": "TRACE_ON_NAVIGATION_UNTIL_TRIGGER_OR_FULL",
      "trigger_name": "session-restore-config",
      "category": "BENCHMARK_NAVIGATION",
      "trigger_delay": 5
    }
  ]
}


Change-Id: Ic9131440db98d260e1979f6cc7dc94cfc1a9c4fb
Reviewed-on: https://chromium-review.googlesource.com/c/1334548
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609345}