[Aura] Reduce frequency of PinchUpdate events.

ui/events/gesture_detection/scale_gesture_detector.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 UI_EVENTS_GESTURE_DETECTION_SCALE_GESTURE_DETECTOR_H_
 #define UI_EVENTS_GESTURE_DETECTION_SCALE_GESTURE_DETECTOR_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "ui/events/gesture_detection/gesture_detection_export.h"
@@ skipping 15 matching lines @@
     GestureDetector::Config gesture_detector_config;
 
     // Minimum accepted value for TouchMajor while scaling (in dips).
     float min_scaling_touch_major;
 
     // Minimum span needed to initiate a scaling gesture (in dips).
     float min_scaling_span;
 
     // Whether double-tap drag scaling is enabled.
     bool quick_scale_enabled;
+
+    // Minimum pinch span change before pinch occurs (in dips). See
+    // crbug.com/373318.
+    float min_pinch_update_span_delta;
   };
 
   class ScaleGestureListener {
    public:
     virtual ~ScaleGestureListener() {}
     virtual bool OnScale(const ScaleGestureDetector& detector,
                          const MotionEvent& e) = 0;
     virtual bool OnScaleBegin(const ScaleGestureDetector& detector,
                               const MotionEvent& e) = 0;
     virtual void OnScaleEnd(const ScaleGestureDetector& detector,
@@ skipping 97 matching lines @@
   bool event_before_or_above_starting_gesture_event_;
 
   scoped_ptr<GestureDetector> gesture_detector_;
 
   DISALLOW_COPY_AND_ASSIGN(ScaleGestureDetector);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURE_DETECTION_SCALE_GESTURE_DETECTOR_H_