Add initial GTK web accessibility framework (new with valgrind fix).

content/browser/accessibility/browser_accessibility_mac.h

Index: content/browser/accessibility/browser_accessibility_mac.h
===================================================================
--- content/browser/accessibility/browser_accessibility_mac.h	(revision 135549)
+++ content/browser/accessibility/browser_accessibility_mac.h	(working copy)
@@ -12,6 +12,7 @@
 
 #include "base/memory/scoped_nsobject.h"
 #include "content/browser/accessibility/browser_accessibility.h"
+#include "content/common/content_export.h"
 
 @class BrowserAccessibilityCocoa;
 
@@ -23,6 +24,8 @@
 
   // Overrides from BrowserAccessibility.
   virtual void DetachTree(std::vector<BrowserAccessibility*>* nodes) OVERRIDE;
+  CONTENT_EXPORT virtual BrowserAccessibilityMac*
+      ToBrowserAccessibilityMac() OVERRIDE;
 
   // The BrowserAccessibilityCocoa associated with us.
   BrowserAccessibilityCocoa* native_view() const {