Fix Mac accessibility window attribute inside child frames.

content/browser/accessibility/browser_accessibility_manager_mac.mm

Index: content/browser/accessibility/browser_accessibility_manager_mac.mm
diff --git a/content/browser/accessibility/browser_accessibility_manager_mac.mm b/content/browser/accessibility/browser_accessibility_manager_mac.mm
index 5fe7dde6a809ccc7181de3131dedd411c5d8783c..604e42008faa45669f06517fad947ed8f2c832f0 100644
--- a/content/browser/accessibility/browser_accessibility_manager_mac.mm
+++ b/content/browser/accessibility/browser_accessibility_manager_mac.mm
@@ -105,6 +105,11 @@ BrowserAccessibilityManager* BrowserAccessibilityManager::Create(
       NULL, initial_tree, delegate, factory);
 }
 
+BrowserAccessibilityManagerMac*
+BrowserAccessibilityManager::ToBrowserAccessibilityManagerMac() {
+  return static_cast<BrowserAccessibilityManagerMac*>(this);
+}
+
 BrowserAccessibilityManagerMac::BrowserAccessibilityManagerMac(
     NSView* parent_view,
     const ui::AXTreeUpdate& initial_tree,