Add touch drag UMA latency metric.

content/browser/renderer_host/input/render_widget_host_latency_tracker.h

Index: content/browser/renderer_host/input/render_widget_host_latency_tracker.h
diff --git a/content/browser/renderer_host/input/render_widget_host_latency_tracker.h b/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
index 633e4e878da43bb4097739ec8f934ec3c227ffd9..143832d60c26fff70bf998cee97388f449711cd8 100644
--- a/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
+++ b/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
@@ -27,18 +27,29 @@ class CONTENT_EXPORT RenderWidgetHostLatencyTracker {
   // Called once after the RenderWidgetHost is fully initialized.
   void Initialize(int routing_id, int process_id);
 
+  void ComputeInputLatencyHistograms(blink::WebInputEvent::Type type,
+                                     int64_t latency_component_id,
+                                     const ui::LatencyInfo& latency,
+                                     bool default_prevented);
+
   // Populates the LatencyInfo with relevant entries for latency tracking.
   // Called when an event is received by the RenderWidgetHost, prior to
   // that event being forwarded to the renderer (via the InputRouter).
   void OnInputEvent(const blink::WebInputEvent& event,
                     ui::LatencyInfo* latency);
 
+  // Populates the LatencyInfo with relevant entries for latency
+  // tracking. Called when an event is about to be forwarded to the renderer.
+  void OnForwardEventToRenderer(const blink::WebInputEvent& event,
+                                ui::LatencyInfo* latency);
+
   // Populates the LatencyInfo with relevant entries for latency tracking, also
   // terminating latency tracking for events that did not trigger rendering and
   // performing relevant UMA latency reporting. Called when an event is ack'ed
   // to the RenderWidgetHost (from the InputRouter).
   void OnInputEventAck(const blink::WebInputEvent& event,
-                       ui::LatencyInfo* latency);
+                       ui::LatencyInfo* latency,
+                       bool defaultPrevented);

[aelias_OOO_until_Jul13, 2016/04/05 18:35:18]

I think it might be useful to have a way to break out those events that were
allowed, but still blocked on the Blink main thread (as opposed to those
processed on the fast path).  Should we make defaultPrevented an enum instead of
a boolean to specify that?

 
   // Populates renderer-created LatencyInfo entries with the appropriate latency
   // component id. Called when the RenderWidgetHost receives a compositor swap
@@ -65,6 +76,12 @@ class CONTENT_EXPORT RenderWidgetHostLatencyTracker {
   int64_t latency_component_id_;
   float device_scale_factor_;
   bool has_seen_first_gesture_scroll_update_;
+  // Whether the current stream of touch events has ever included more than one
+  // touch point.
+  bool multi_finger_gesture_;
+  // Whether the touch start for the current stream of touch events had its
+  // default action prevented. Only valid for single finger gestures.
+  bool touch_start_default_prevented_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostLatencyTracker);
 };