Enable PDF accessibility when RenderFrame's accessibility mode changes.

content/public/renderer/render_frame_observer.h

Index: content/public/renderer/render_frame_observer.h
diff --git a/content/public/renderer/render_frame_observer.h b/content/public/renderer/render_frame_observer.h
index a48fa5422eba0a8e41b9ffadb7fd05e6a3faea1c..8357949e2078896ae23b21eed1cf3d3e4eeab210 100644
--- a/content/public/renderer/render_frame_observer.h
+++ b/content/public/renderer/render_frame_observer.h
@@ -122,6 +122,9 @@ class CONTENT_EXPORT RenderFrameObserver : public IPC::Listener,
   // Called when the focused node has changed to |node|.
   virtual void FocusedNodeChanged(const blink::WebNode& node) {}
 
+  // Called when accessibility is enabled or disabled.
+  virtual void AccessibilityModeChanged() {}
+
   // IPC::Listener implementation.
   bool OnMessageReceived(const IPC::Message& message) override;