Add initial GTK web accessibility framework.

content/browser/accessibility/browser_accessibility.cc

Index: content/browser/accessibility/browser_accessibility.cc
===================================================================
--- content/browser/accessibility/browser_accessibility.cc	(revision 134706)
+++ content/browser/accessibility/browser_accessibility.cc	(working copy)
@@ -15,9 +15,9 @@
 typedef WebAccessibility::IntAttribute IntAttribute;
 typedef WebAccessibility::StringAttribute StringAttribute;
 
-#if (defined(OS_POSIX) && !defined(OS_MACOSX)) || defined(USE_AURA)
-// There's no OS-specific implementation of BrowserAccessibilityManager
-// on Unix, so just instantiate the base class.
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(TOOLKIT_GTK)
+// We have implementations of BrowserAccessibility on Win, Mac,
+// and GTK. If we have anything else, just instantiate the base class.
 // static
 BrowserAccessibility* BrowserAccessibility::Create() {
   return new BrowserAccessibility();