Implement Middle Click Autoscroll on all platforms not just Windows

third_party/WebKit/Source/core/input/ScrollManager.h

 // Copyright 2016 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 ScrollManager_h
 #define ScrollManager_h
 
 #include "core/CoreExport.h"
 #include "core/page/EventWithHitTestResults.h"
 #include "platform/PlatformEvent.h"
@@ skipping 25 matching lines @@
 class CORE_EXPORT ScrollManager {
     WTF_MAKE_NONCOPYABLE(ScrollManager);
     DISALLOW_NEW();
 public:
     explicit ScrollManager(LocalFrame*);
     ~ScrollManager();
     DECLARE_TRACE();
 
     void clear();
 
-    bool panScrollInProgress() const;
+    bool middleClickAutoscrollInProgress() const;
     AutoscrollController* autoscrollController() const;
     void stopAutoscroll();
 
     // Performs a chaining logical scroll, within a *single* frame, starting
     // from either a provided starting node or a default based on the focused or
     // most recently clicked node, falling back to the frame.
     // Returns true if the scroll was consumed.
     // direction - The logical direction to scroll in. This will be converted to
     //             a physical direction for each LayoutBox we try to scroll
     //             based on that box's writing mode.
@@ skipping 80 matching lines @@
 
     Member<PaintLayerScrollableArea> m_resizeScrollableArea;
 
     LayoutSize m_offsetFromResizeCorner; // In the coords of m_resizeScrollableArea.
 
 };
 
 } // namespace blink
 
 #endif // ScrollManager_h