Log First User Interaction in Page Load Metrics

content/browser/renderer_host/input/render_widget_host_latency_tracker.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_RENDER_WIDGET_HOST_LATENCY_TRACKER_H_
 #define CONTENT_BROWSER_RENDERER_HOST_INPUT_RENDER_WIDGET_HOST_LATENCY_TRACKER_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 42 matching lines @@
   // WebInputEvent coordinates are in DPIs, while LatencyInfo expects
   // coordinates in device pixels.
   void set_device_scale_factor(float device_scale_factor) {
     device_scale_factor_ = device_scale_factor;
   }
 
   // Returns the ID that uniquely describes this component to the latency
   // subsystem.
   int64_t latency_component_id() const { return latency_component_id_; }
 
+  // Add/remove a callback that can handle input events routed to latency
+  // tracker.
+  typedef base::Callback<bool(const blink::WebInputEvent&)> InputEventCallback;
+  void AddInputEventCallback(const InputEventCallback& callback);
+  void RemoveInputEventCallback(const InputEventCallback& callback);
+
  private:
   int64_t last_event_id_;
   int64_t latency_component_id_;
   float device_scale_factor_;
   bool has_seen_first_gesture_scroll_update_;
+  std::vector<InputEventCallback> input_event_callbacks_;

[Bryan McQuade, 2016/05/25 21:32:30]

can you use base::ObserverList here? the comments note that it's safer to use
than vector<> since clients can safely remove themselves as observers during
iteration, whereas that doesn't work correctly with vector<>s.

 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostLatencyTracker);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_INPUT_RENDER_WIDGET_HOST_LATENCY_TRACKER_H_