Components: fix dead observer notification crash

When destructing ComponentsHandler, RemoveObserver() was not being
called in all cases, and therefore ComponentUpdaterService had a dead
pointer dangling until it next notifies.

R=groby@chromium.org

Fixed: 1038846
Change-Id: I2ec97e41d062ddd788c7caa6b236b4bfdb56b6be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1987282
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Rachel Blum <groby@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728365}

chrome/browser/ui/webui/components/components_handler.cc

@@ -26,16 +26,11 @@ void ComponentsHandler::RegisterMessages() {
 }
 
 void ComponentsHandler::OnJavascriptAllowed() {
-  component_updater::ComponentUpdateService* cus =
-      g_browser_process->component_updater();
-  cus->AddObserver(this);
+  observer_.Add(g_browser_process->component_updater());
 }
 
 void ComponentsHandler::OnJavascriptDisallowed() {
-  component_updater::ComponentUpdateService* cus =
-      g_browser_process->component_updater();
-  if (cus)
-    cus->RemoveObserver(this);
+  observer_.RemoveAll();
 }
 
 void ComponentsHandler::HandleRequestComponentsData(
@@ -142,7 +137,7 @@ base::string16 ComponentsHandler::ServiceStatusToString(
 void ComponentsHandler::OnDemandUpdate(const std::string& component_id) {
   component_updater::ComponentUpdateService* cus =
       g_browser_process->component_updater();
-  if (cus) {
+  if (cus) {  // TODO(dbeam): can this return nullptr if called from UI thread?
     cus->GetOnDemandUpdater().OnDemandUpdate(
         component_id, component_updater::OnDemandUpdater::Priority::FOREGROUND,
         component_updater::Callback());

chrome/browser/ui/webui/components/components_handler.h

@@ -8,10 +8,10 @@
 #include <memory>
 #include <string>
 
+#include "base/scoped_observer.h"
 #include "base/strings/string16.h"
 #include "components/component_updater/component_updater_service.h"
 #include "components/update_client/update_client.h"
-#include "content/public/browser/notification_registrar.h"
 #include "content/public/browser/web_ui_message_handler.h"
 
 namespace base {
@@ -48,7 +48,9 @@ class ComponentsHandler : public content::WebUIMessageHandler,
   static std::unique_ptr<base::ListValue> LoadComponents();
   static void OnDemandUpdate(const std::string& component_id);
 
-  content::NotificationRegistrar registrar_;
+  ScopedObserver<component_updater::ComponentUpdateService,
+                 component_updater::ComponentUpdateService::Observer>
+      observer_{this};
 };
 
 #endif  // CHROME_BROWSER_UI_WEBUI_COMPONENTS_COMPONENTS_HANDLER_H_