Adding MeterRole in blink to support MSAA+IA2 and AX roles for meter tag in Chromium.

Source/core/accessibility/AXObject.h

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * 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
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 106 matching lines @@
     MathRole,
     MatteRole,
     MenuBarRole,
     MenuButtonRole,
     MenuItemRole,
     MenuItemCheckBoxRole,
     MenuItemRadioRole,
     MenuListOptionRole,
     MenuListPopupRole,
     MenuRole,
+    MeterRole,
     NavigationRole,
     NoneRole,
     NoteRole,
     OutlineRole,
     ParagraphRole,
     PopUpButtonRole,
     PresentationalRole,
     ProgressIndicatorRole,
     RadioButtonRole,
     RadioGroupRole,
@@ skipping 441 matching lines @@
 
     bool m_detached;
 };
 
 #define DEFINE_AX_OBJECT_TYPE_CASTS(thisType, predicate) \
     DEFINE_TYPE_CASTS(thisType, AXObject, object, object->predicate, object.predicate)
 
 } // namespace blink
 
 #endif // AXObject_h