Add UMA measuring pan scrolling usage.

third_party/WebKit/Source/core/page/AutoscrollController.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 205 matching lines @@
 void AutoscrollController::startPanScrolling(LayoutBox* scrollable, const IntPoint& lastKnownMousePosition)
 {
     // We don't want to trigger the autoscroll or the panScroll if it's already active
     if (m_autoscrollType != NoAutoscroll)
         return;
 
     m_autoscrollType = AutoscrollForPan;
     m_autoscrollLayoutObject = scrollable;
     m_panScrollStartPos = lastKnownMousePosition;
 
+    UseCounter::count(m_page->mainFrame(), UseCounter::PanScrollingStart);
     startAutoscroll();
 }
 #else
 bool AutoscrollController::panScrollInProgress() const
 {
     return false;
 }
 #endif
 
 // FIXME: This would get get better animation fidelity if it used the monotonicFrameBeginTime instead
@@ skipping 74 matching lines @@
         view->setCursor(eastPanningCursor());
     } else if (west) {
         view->setCursor(westPanningCursor());
     } else {
         view->setCursor(middlePanningCursor());
     }
 }
 #endif
 
 } // namespace blink