Revert of Remove deprecated extension notification from content_settings...

Revert of Remove deprecated extension notification from content_settings (patchset #6 id:240001 of https://codereview.chromium.org/276573011/)

Reason for revert:
Becuase this CL seems to break linux asan test
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_asan_rel/builds/882

Original issue's description:
> remove deprecated extension notification from content_settings.
>
> Use ExtensionRegistryObserver instead.
>
> R=bauerb@chromium.org
> BUG=354046, 354458
> TEST=unit_tests
>
> Committed: https://crrev.com/76ad58487578183de69a8caeb48b8a22dccad652
> Cr-Commit-Position: refs/heads/master@{#294768}

TBR=bauerb@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=354046, 354458

Review URL: https://codereview.chromium.org/553263004

Cr-Commit-Position: refs/heads/master@{#294773}

chrome/browser/content_settings/content_settings_internal_extension_provider.h

@@ -12,21 +12,18 @@
 #include "components/content_settings/core/common/content_settings.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
-#include "extensions/browser/extension_registry_observer.h"
 
 class ExtensionService;
 
 namespace extensions {
 class Extension;
-class ExtensionRegistry;
 }
 
 namespace content_settings {
 
 // A content settings provider which disables certain plugins for platform apps.
 class InternalExtensionProvider : public ObservableProvider,
-                                  public content::NotificationObserver,
-                                  public extensions::ExtensionRegistryObserver {
+                            public content::NotificationObserver {
  public:
   explicit InternalExtensionProvider(ExtensionService* extension_service);
 
@@ -54,16 +51,6 @@ class InternalExtensionProvider : public ObservableProvider,
   virtual void Observe(int type,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
-
-  // extensions::ExtensionRegistryObserver implementation.
-  virtual void OnExtensionLoaded(
-      content::BrowserContext* browser_context,
-      const extensions::Extension* extension) OVERRIDE;
-  virtual void OnExtensionUnloaded(
-      content::BrowserContext* browser_context,
-      const extensions::Extension* extension,
-      extensions::UnloadedExtensionInfo::Reason reason) OVERRIDE;
-
  private:
   void SetContentSettingForExtension(const extensions::Extension* extension,
                                      ContentSetting setting);
@@ -78,8 +65,6 @@ class InternalExtensionProvider : public ObservableProvider,
   mutable base::Lock lock_;
   scoped_ptr<content::NotificationRegistrar> registrar_;
 
-  extensions::ExtensionRegistry* extension_registry_;  // Not owned.
-
   DISALLOW_COPY_AND_ASSIGN(InternalExtensionProvider);
 };