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

content/browser/accessibility/browser_accessibility_manager_mac.mm

Index: content/browser/accessibility/browser_accessibility_manager_mac.mm
===================================================================
--- content/browser/accessibility/browser_accessibility_manager_mac.mm	(revision 135142)
+++ content/browser/accessibility/browser_accessibility_manager_mac.mm	(working copy)
@@ -29,6 +29,10 @@
 void BrowserAccessibilityManagerMac::NotifyAccessibilityEvent(
     int type,
     BrowserAccessibility* node) {
+  BrowserAccessibilityCocoa* native_node = node->ToBrowserAccessibilityCocoa();
+  if (!native_node)
+    return;
+
   // Refer to AXObjectCache.mm (webkit).
   NSString* event_id = @"";
   switch (type) {
@@ -98,7 +102,5 @@
       event_id = NSAccessibilityValueChangedNotification;
       break;
   }
-  BrowserAccessibilityCocoa* native_node = node->toBrowserAccessibilityCocoa();
-  DCHECK(native_node);
   NSAccessibilityPostNotification(native_node, event_id);
 }