[Tracing] Use TracingController as the interface for tracing start/stop

Currently some places use TraceLog::SetEnabled directly,
e.g. startup and shutdown tracing. We want to make them use
TracingController's interface, so that tracing is started
in the same standard way. Later when we start to support
multiple tracing methods with clock sync in Chrome, those
places will automatically benefit from it.

For startup tracing, the place that starts tracing is
before threads are ready. So we have to use
TraceLog::SetEnabled first. And right after threads are
ready, we call TracingController::StartTracing to enable
other tracing agents. Startup tracing should be the only
place that uses TraceLog::SetEnabled for the constraint
mentioned. Other places should use TracingController's
interface.

Doc on how startup tracing works:
https://docs.google.com/document/d/1A26E8bRJNiX7lmjyYcPGdNDJWcQ7cnOUbaXRpg8S8v0/edit?usp=sharing

TEST normal startup tracing with default configuration:
$ ./out/Debug/chrome --trace-config-file

TEST startup tracing for memory infra:
$ cat > /tmp/trace.config
{
  "startup_duration": 5,
  "result_file": "/tmp/trace.json",
  "trace_config": {
    "included_categories": ["disabled-by-default-memory-infra"],
    "excluded_categories": ["*"],
    "memory_dump_config": {
      "triggers": [
        {"mode":"light", "periodic_interval_ms": 50},
        {"mode":"detailed", "periodic_interval_ms": 1000}
      ]
    }
  }
}
$ ./out/Debug/chrome --no-sandbox --trace-config-file=/tmp/trace.config

TEST startup tracing on Android
$ adb push /tmp/trace.config /data/local/chrome-trace-config.json
Run Chrome on Android and the result file is in /sdcard/Download/.

TEST shutdown tracing
$ ./out/Debug/chrome --trace-shutdown

BUG=542390

Review URL: https://codereview.chromium.org/1423003003

Cr-Commit-Position: refs/heads/master@{#360871}

1 files changed, 6 insertions, 0 deletions

diff --git a/content/renderer/devtools/devtools_agent.cc b/content/renderer/devtools/devtools_agent.cc
index 93dfd84..3b2f47e 100644
--- a/content/renderer/devtools/devtools_agent.cc
+++ b/content/renderer/devtools/devtools_agent.cc
@@ -131,6 +131,12 @@ void DevToolsAgent::didExitDebugLoop() {
 }
 
 void DevToolsAgent::enableTracing(const WebString& category_filter) {
+  // Tracing is already started by DevTools TracingHandler::Start for the
+  // renderer target in the browser process. It will eventually start tracing in
+  // the renderer process via IPC. But we still need a redundant
+  // TraceLog::SetEnabled call here for
+  // InspectorTracingAgent::emitMetadataEvents(), at which point, we are not
+  // sure if tracing is already started in the renderer process.
   TraceLog* trace_log = TraceLog::GetInstance();
   trace_log->SetEnabled(
       base::trace_event::TraceConfig(category_filter.utf8(), ""),