Add initial GTK web accessibility framework (third attempt).

content/browser/accessibility/browser_accessibility.cc

Index: content/browser/accessibility/browser_accessibility.cc
===================================================================
--- content/browser/accessibility/browser_accessibility.cc	(revision 135807)
+++ content/browser/accessibility/browser_accessibility.cc	(working copy)
@@ -15,9 +15,11 @@
 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_MACOSX) && \
+    !(defined(OS_WIN) && !defined(USE_AURA)) && \
+    !defined(TOOLKIT_GTK)
+// We have subclassess of BrowserAccessibility on Mac, Linux/GTK,
+// and non-Aura Win. For any other platform, instantiate the base class.
 // static
 BrowserAccessibility* BrowserAccessibility::Create() {
   return new BrowserAccessibility();
@@ -79,6 +81,10 @@
   PreInitialize();
 }
 
+bool BrowserAccessibility::IsNative() const {
+  return false;
+}
+
 void BrowserAccessibility::AddChild(BrowserAccessibility* child) {
   children_.push_back(child);
 }