Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/accessibility/renderer_accessibility.h

Index: content/renderer/accessibility/renderer_accessibility.h
diff --git a/content/renderer/accessibility/renderer_accessibility.h b/content/renderer/accessibility/renderer_accessibility.h
index 91bcd553afc1f64701ac09da1ce24fb8beb9e157..ecd1dae5b2e609652f71b8ce0dc9452181da2b6c 100644
--- a/content/renderer/accessibility/renderer_accessibility.h
+++ b/content/renderer/accessibility/renderer_accessibility.h
@@ -100,6 +100,9 @@ class CONTENT_EXPORT RendererAccessibility : public RenderFrameObserver {
   RenderFrameImpl* render_frame_;
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   // Handlers for messages from the browser to the renderer.
   void OnDoDefaultAction(int acc_obj_id);
   void OnEventsAck();