Revert of Remove deprecated extension notification from ProcessManager.

extensions/browser/process_manager.cc

Index: extensions/browser/process_manager.cc
diff --git a/extensions/browser/process_manager.cc b/extensions/browser/process_manager.cc
index 38c476e016dcb3bdc5d988b5c4f8b879f8e38de0..c3bcd50ae425ff35c102616c72f73461c18ace31 100644
--- a/extensions/browser/process_manager.cc
+++ b/extensions/browser/process_manager.cc
@@ -247,9 +247,14 @@
       weak_ptr_factory_(this) {
   // ExtensionRegistry is shared between incognito and regular contexts.
   DCHECK_EQ(original_context, extension_registry_->browser_context());
-  extension_registry_->AddObserver(this);
   registrar_.Add(this,
                  extensions::NOTIFICATION_EXTENSIONS_READY_DEPRECATED,
+                 content::Source<BrowserContext>(original_context));
+  registrar_.Add(this,
+                 extensions::NOTIFICATION_EXTENSION_LOADED_DEPRECATED,
+                 content::Source<BrowserContext>(original_context));
+  registrar_.Add(this,
+                 extensions::NOTIFICATION_EXTENSION_UNLOADED_DEPRECATED,
                  content::Source<BrowserContext>(original_context));
   registrar_.Add(this,
                  extensions::NOTIFICATION_EXTENSION_HOST_DESTROYED,
@@ -286,7 +291,6 @@
 }
 
 ProcessManager::~ProcessManager() {
-  extension_registry_->RemoveObserver(this);
   CloseBackgroundHosts();
   DCHECK(background_hosts_.empty());
   content::DevToolsAgentHost::RemoveAgentStateCallback(devtools_callback_);
@@ -684,6 +688,33 @@
       break;
     }
 
+    case extensions::NOTIFICATION_EXTENSION_LOADED_DEPRECATED: {
+      BrowserContext* context = content::Source<BrowserContext>(source).ptr();
+      ExtensionSystem* system = ExtensionSystem::Get(context);
+      if (system->ready().is_signaled()) {
+        // The extension system is ready, so create the background host.
+        const Extension* extension =
+            content::Details<const Extension>(details).ptr();
+        CreateBackgroundHostForExtensionLoad(this, extension);
+      }
+      break;
+    }
+
+    case extensions::NOTIFICATION_EXTENSION_UNLOADED_DEPRECATED: {
+      const Extension* extension =
+          content::Details<UnloadedExtensionInfo>(details)->extension;
+      for (ExtensionHostSet::iterator iter = background_hosts_.begin();
+           iter != background_hosts_.end(); ++iter) {
+        ExtensionHost* host = *iter;
+        if (host->extension_id() == extension->id()) {
+          CloseBackgroundHost(host);
+          break;
+        }
+      }
+      UnregisterExtension(extension->id());
+      break;
+    }
+
     case extensions::NOTIFICATION_EXTENSION_HOST_DESTROYED: {
       ExtensionHost* host = content::Details<ExtensionHost>(details).ptr();
       if (background_hosts_.erase(host)) {
@@ -747,31 +778,6 @@
     default:
       NOTREACHED();
   }
-}
-
-void ProcessManager::OnExtensionLoaded(content::BrowserContext* browser_context,
-                                       const extensions::Extension* extension) {
-  ExtensionSystem* system = ExtensionSystem::Get(browser_context);
-  if (system->ready().is_signaled()) {
-    // The extension system is ready, so create the background host.
-    CreateBackgroundHostForExtensionLoad(this, extension);
-  }
-}
-
-void ProcessManager::OnExtensionUnloaded(
-    content::BrowserContext* browser_context,
-    const extensions::Extension* extension,
-    extensions::UnloadedExtensionInfo::Reason reason) {
-  for (ExtensionHostSet::iterator iter = background_hosts_.begin();
-       iter != background_hosts_.end();
-       ++iter) {
-    ExtensionHost* host = *iter;
-    if (host->extension_id() == extension->id()) {
-      CloseBackgroundHost(host);
-      break;
-    }
-  }
-  UnregisterExtension(extension->id());
 }
 
 void ProcessManager::OnDevToolsStateChanged(