[Aura] Reduce frequency of PinchUpdate events.

ui/events/gesture_detection/gesture_provider.cc

Index: ui/events/gesture_detection/gesture_provider.cc
diff --git a/ui/events/gesture_detection/gesture_provider.cc b/ui/events/gesture_detection/gesture_provider.cc
index 7607f08d8765a7fbc02a96f7eb471af5f813efcb..55bc37010402aad0021a62760f0a07d5d378ca89 100644
--- a/ui/events/gesture_detection/gesture_provider.cc
+++ b/ui/events/gesture_detection/gesture_provider.cc
@@ -130,7 +130,8 @@ class GestureProvider::ScaleGestureListenerImpl
       : scale_gesture_detector_(config, this),
         provider_(provider),
         ignore_multitouch_events_(false),
-        pinch_event_sent_(false) {}
+        pinch_event_sent_(false),
+        min_pinch_update_span_delta_(config.min_pinch_update_span_delta) {}
 
   bool OnTouchEvent(const MotionEvent& event) {
     // TODO: Need to deal with multi-touch transition.
@@ -182,6 +183,11 @@ class GestureProvider::ScaleGestureListenerImpl
                                     GetBoundingBox(e)));
     }
 
+    if (std::abs(detector.GetCurrentSpan() - detector.GetPreviousSpan()) <
+        min_pinch_update_span_delta_) {
+      return false;
+    }
+
     float scale = detector.GetScaleFactor();
     if (scale == 1)
       return true;
@@ -247,6 +253,10 @@ class GestureProvider::ScaleGestureListenerImpl
   // Whether any pinch zoom event has been sent to native.
   bool pinch_event_sent_;
 
+  // The minimum change in span required before this is considered a pinch. See
+  // crbug.com/373318.
+  float min_pinch_update_span_delta_;
+
   DISALLOW_COPY_AND_ASSIGN(ScaleGestureListenerImpl);
 };