Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/public/renderer/render_frame_observer.cc

Index: content/public/renderer/render_frame_observer.cc
diff --git a/content/public/renderer/render_frame_observer.cc b/content/public/renderer/render_frame_observer.cc
index 19d77d01f0e0aeed354beff38cf40dbd708a1d6c..9e26c91976b5519cc104132002b3d1997aa8d0fc 100644
--- a/content/public/renderer/render_frame_observer.cc
+++ b/content/public/renderer/render_frame_observer.cc
@@ -29,10 +29,6 @@ RenderFrameObserver::~RenderFrameObserver() {
   }
 }
 
-void RenderFrameObserver::OnDestruct() {
-  delete this;
-}
-
 bool RenderFrameObserver::OnMessageReceived(const IPC::Message& message) {
   return false;
 }