content: Remove usage of NOTIFICATION_WEB_CONTENTS_DESTROYED from content

content/shell/shell.cc

Index: content/shell/shell.cc
diff --git a/content/shell/shell.cc b/content/shell/shell.cc
index ff14a7c6af0501a95839aa0d850cbbf0ee1a1737..7cf568e0676fd148a362863bc29ea6091ea1d6b0 100644
--- a/content/shell/shell.cc
+++ b/content/shell/shell.cc
@@ -43,7 +43,8 @@ base::Callback<void(Shell*)> Shell::shell_created_callback_;
 bool Shell::quit_message_loop_ = true;
 
 Shell::Shell(WebContents* web_contents)
-    : devtools_frontend_(NULL),
+    : WebContentsObserver(NULL),  // Watch DevTools WebContents, not the above.
+      devtools_frontend_(NULL),
       is_fullscreen_(false),
       window_(NULL),
       url_edit_view_(NULL),
@@ -192,19 +193,14 @@ void Shell::ShowDevTools() {
     return;
   }
   devtools_frontend_ = ShellDevToolsFrontend::Show(web_contents());
-  registrar_.Add(this,
-                 NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                 Source<WebContents>(
-                     devtools_frontend_->frontend_shell()->web_contents()));
+  WebContentsObserver::Observe(
+      devtools_frontend_->frontend_shell()->web_contents());
 }
 
 void Shell::CloseDevTools() {
   if (!devtools_frontend_)
     return;
-  registrar_.Remove(this,
-                    NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                    Source<WebContents>(
-                        devtools_frontend_->frontend_shell()->web_contents()));
+  WebContentsObserver::Observe(NULL);
   devtools_frontend_->Close();
   devtools_frontend_ = NULL;
 }
@@ -310,6 +306,11 @@ void Shell::DeactivateContents(WebContents* contents) {
   contents->GetRenderViewHost()->Blur();
 }
 
+void Shell::WebContentsDestroyed(WebContents* web_contents) {
+  devtools_frontend_ = NULL;
+  WebContentsObserver::Observe(NULL);
+}
+
 void Shell::Observe(int type,
                     const NotificationSource& source,
                     const NotificationDetails& details) {
@@ -321,9 +322,6 @@ void Shell::Observe(int type,
       string16 text = title->first->GetTitle();
       PlatformSetTitle(text);
     }
-  } else if (type == NOTIFICATION_WEB_CONTENTS_DESTROYED) {
-    devtools_frontend_ = NULL;
-    registrar_.Remove(this, NOTIFICATION_WEB_CONTENTS_DESTROYED, source);
   } else {
     NOTREACHED();
   }