Clear StartTracingDoneCallback after usage

Currently StartTracingDoneCallback is not cleared after it is
used. This CL fixes it.

BUG=542390

Review URL: https://codereview.chromium.org/1612583002

Cr-Commit-Position: refs/heads/master@{#370811}

content/browser/tracing/tracing_controller_impl.cc

@@ -270,6 +270,8 @@ void TracingControllerImpl::OnStartAgentTracingDone(
 
   if (!callback.is_null())
     callback.Run();
+
+  start_tracing_done_callback_.Reset();
 }
 
 bool TracingControllerImpl::StopTracing(