Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

extensions/renderer/extension_helper.cc

Index: extensions/renderer/extension_helper.cc
diff --git a/extensions/renderer/extension_helper.cc b/extensions/renderer/extension_helper.cc
index 4d3bd2711dc58d0465f69f5db1605b055fdffa8a..f1371fb27c3824a0db41dfc53d49fdb613497510 100644
--- a/extensions/renderer/extension_helper.cc
+++ b/extensions/renderer/extension_helper.cc
@@ -41,6 +41,10 @@ bool ExtensionHelper::OnMessageReceived(const IPC::Message& message) {
   return handled;
 }
 
+void ExtensionHelper::OnDestruct() {
+  delete this;
+}
+
 void ExtensionHelper::DraggableRegionsChanged(blink::WebFrame* frame) {
   blink::WebVector<blink::WebDraggableRegion> webregions =
       frame->document().draggableRegions();