Adopt "QuickScale" double-tap drag zoom code in the GestureProvider

ui/events/gesture_detection/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_GESTURE_DETECTOR_H_
 #define UI_EVENTS_GESTURE_DETECTION_GESTURE_DETECTOR_H_
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/events/gesture_detection/gesture_detection_export.h"
@@ skipping 70 matching lines @@
                   DoubleTapListener* optional_double_tap_listener);
   ~GestureDetector();
 
   bool OnTouchEvent(const MotionEvent& ev);
 
   void set_doubletap_listener(DoubleTapListener* double_tap_listener) {
     DCHECK(!is_double_tapping_ || double_tap_listener_);
     double_tap_listener_ = double_tap_listener;
   }
 
+  bool has_doubletap_listener() const { return double_tap_listener_ != NULL; }
+
+  bool is_double_tapping() const { return is_double_tapping_; }
+
   void set_is_longpress_enabled(bool is_longpress_enabled) {
     is_longpress_enabled_ = is_longpress_enabled;
   }
 
   bool is_longpress_enabled() const { return is_longpress_enabled_; }
 
  private:
   void Init(const Config& config);
   void OnShowPressTimeout();
   void OnLongPressTimeout();
@@ skipping 38 matching lines @@
 
   // Determines speed during touch scrolling.
   VelocityTrackerState velocity_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(GestureDetector);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURE_DETECTION_GESTURE_DETECTOR_H_