[ash-md] Adds support for gesture to move selection in overview mode

ash/wm/gestures/overview_gesture_handler.h

 // Copyright 2013 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 ASH_WM_GESTURES_OVERVIEW_GESTURE_HANDLER_H_
 #define ASH_WM_GESTURES_OVERVIEW_GESTURE_HANDLER_H_
 
+#include "ash/ash_export.h"
 #include "base/macros.h"
 
 namespace ui {
 class ScrollEvent;
 }
 
 namespace ash {
 
 // This handles 3-finger touchpad scroll events to enter/exit overview mode.
-class OverviewGestureHandler {
+class ASH_EXPORT OverviewGestureHandler {
  public:
   OverviewGestureHandler();
   virtual ~OverviewGestureHandler();
 
   // Processes a scroll event and may start overview. Returns true if the event
   // has been handled and should not be processed further, false otherwise.
   bool ProcessScrollEvent(const ui::ScrollEvent& event);
 
  private:
-  // The total distance scrolled with three fingers.
+  friend class OverviewGestureHandlerTest;
+
+  // The total distance scrolled with three fingers up to the point when an
+  // action is triggered. When the action (enter / exit overview mode or move
+  // selection in overview) is triggered those values are reset to zero.
   float scroll_x_;
   float scroll_y_;
 
+  // The threshold before engaging overview with a touchpad three-finger scroll.
+  static const float vertical_threshold_pixels_;
+
+  // The threshold before moving selector horizontally when using a touchpad
+  // two or three-finger scroll.
+  static const float horizontal_threshold_pixels_;
+
   DISALLOW_COPY_AND_ASSIGN(OverviewGestureHandler);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_GESTURES_OVERVIEW_GESTURE_HANDLER_H_