[Aura] Reduce frequency of PinchUpdate events.

ui/events/gesture_detection/scale_gesture_detector.cc

 // 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.
 
 #include "ui/events/gesture_detection/scale_gesture_detector.h"
 
 #include <limits.h>
 #include <cmath>
 
 #include "base/float_util.h"
@@ skipping 15 matching lines @@
 
 const float kScaleFactor = .5f;
 
 }  // namespace
 
 // Note: These constants were taken directly from the default (unscaled)
 // versions found in Android's ViewConfiguration.
 ScaleGestureDetector::Config::Config()
     : min_scaling_touch_major(48),
       min_scaling_span(200),
-      quick_scale_enabled(true) {}
+      quick_scale_enabled(true),
+      min_pinch_update_span_delta(0) {
+}
 
 ScaleGestureDetector::Config::~Config() {}
 
 bool ScaleGestureDetector::SimpleScaleGestureListener::OnScale(
     const ScaleGestureDetector&, const MotionEvent&) {
   return false;
 }
 
 bool ScaleGestureDetector::SimpleScaleGestureListener::OnScaleBegin(
     const ScaleGestureDetector&, const MotionEvent&) {
@@ skipping 318 matching lines @@
 
 void ScaleGestureDetector::ClearTouchHistory() {
   touch_upper_ = std::numeric_limits<float>::quiet_NaN();
   touch_lower_ = std::numeric_limits<float>::quiet_NaN();
   touch_history_last_accepted_ = std::numeric_limits<float>::quiet_NaN();
   touch_history_direction_ = 0;
   touch_history_last_accepted_time_ = base::TimeTicks();
 }
 
 }  // namespace ui