Slow Reports - Add message filter to trigger background traces from child processes.

content/public/browser/background_tracing_manager.h

Index: content/public/browser/background_tracing_manager.h
diff --git a/content/public/browser/background_tracing_manager.h b/content/public/browser/background_tracing_manager.h
index 5be3d611c94acf896c826e295ea56f3c7a3cc2bc..94250d3daa51672af98e49aafa34c0b34879c3b8 100644
--- a/content/public/browser/background_tracing_manager.h
+++ b/content/public/browser/background_tracing_manager.h
@@ -96,6 +96,8 @@ class BackgroundTracingManager {
   virtual void FireTimerForTesting() = 0;
   virtual bool HasActiveScenarioForTesting() = 0;
 
+  virtual void OnHistogramTrigger(const std::string& histogram_name) = 0;
+
  protected:
   virtual ~BackgroundTracingManager() {}
 };