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

content/browser/accessibility/browser_accessibility.h

Index: content/browser/accessibility/browser_accessibility.h
===================================================================
--- content/browser/accessibility/browser_accessibility.h	(revision 135142)
+++ content/browser/accessibility/browser_accessibility.h	(working copy)
@@ -20,6 +20,8 @@
 @class BrowserAccessibilityCocoa;
 #elif defined(OS_WIN)
 class BrowserAccessibilityWin;
+#elif defined(TOOLKIT_GTK)
+class BrowserAccessibilityGtk;
 #endif
 
 using webkit_glue::WebAccessibility;
@@ -188,9 +190,11 @@
   int32 ref_count() const { return ref_count_; }
 
 #if defined(OS_MACOSX) && __OBJC__
-  BrowserAccessibilityCocoa* toBrowserAccessibilityCocoa();
+  virtual BrowserAccessibilityCocoa* ToBrowserAccessibilityCocoa();
 #elif defined(OS_WIN)
-  BrowserAccessibilityWin* toBrowserAccessibilityWin();
+  virtual BrowserAccessibilityWin* ToBrowserAccessibilityWin();
+#elif defined(TOOLKIT_GTK)
+  virtual BrowserAccessibilityGtk* ToBrowserAccessibilityGtk();
 #endif
 
   // Retrieve the value of a bool attribute from the bool attribute