Adds a callback to TracingAgent::StartAgentTracing()

content/browser/tracing/power_tracing_agent.cc

Index: content/browser/tracing/power_tracing_agent.cc
diff --git a/content/browser/tracing/power_tracing_agent.cc b/content/browser/tracing/power_tracing_agent.cc
index 6bf47e66bd2993cf1bb01be41b184d878adb8772..4a3541d81c7bd2e617643a21f64fc889aaf62045 100644
--- a/content/browser/tracing/power_tracing_agent.cc
+++ b/content/browser/tracing/power_tracing_agent.cc
@@ -4,6 +4,7 @@
 
 #include "base/lazy_instance.h"
 #include "base/memory/singleton.h"
+#include "base/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event_impl.h"
 #include "content/browser/tracing/battor_power_trace_provider.h"
 #include "content/browser/tracing/power_tracing_agent.h"
@@ -37,23 +38,27 @@ std::string PowerTracingAgent::GetTraceEventLabel() {
   return kPowerTraceLabel;
 }
 
-bool PowerTracingAgent::StartAgentTracing(
-    const base::trace_event::TraceConfig& trace_config) {
+void PowerTracingAgent::StartAgentTracing(
+    const base::trace_event::TraceConfig& trace_config,
+    const StartAgentTracingCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   // TODO(charliea): When system tracing is enabled in about://tracing, it will
   // trigger power tracing. We need a way of checking if BattOr is connected.
   // Currently, IsConnected() always returns false, so that we do not include
   // BattOr trace until it is hooked up.
-  if (!battor_trace_provider_->IsConnected())
-    return false;
-
+  if (!battor_trace_provider_->IsConnected()) {

[Zhen Wang, 2016/01/25 20:35:32]

This is still a sync call here. Will this be updated to async call in later CL?

[charliea (OOO until 10-5), 2016/01/25 21:15:24]

All interaction with the serial classes needs to be done on the IO thread.
However, the IsConnected() call, once on the IO thread, is a synchronous call. I
think that we're going to have 3 main StartTracing functions in this class all
together: StartTracing (on UI thread), StartTracingOnIOThread, and
IsConnected(). StartTracing will defer to StartTracingOnIOThread, which will in
turn call IsConnected.

That was really complicated to explain, but if it doesn't make sense, I think it
should be a lot more clear w/ the next CL.

+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::Bind(callback, GetTracingAgentName(), false));
+    return;
+  }
   thread_.Start();
 
   thread_.task_runner()->PostTask(
       FROM_HERE,
       base::Bind(&PowerTracingAgent::TraceOnThread, base::Unretained(this)));
-  return true;
+  base::ThreadTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE, base::Bind(callback, GetTracingAgentName(), true));
 }
 
 void PowerTracingAgent::StopAgentTracing(