Update ScopedObserver declarations in extensions directories.

This covers //c/b/ui/extensions and //c/b/extensions

These were found by attempting to compile the chrome target
on Linux.

Note that BrowserList/Observer can't be used with ScopedObserver
because Add/RemoveObserver are statics.

Bug: 998625
Change-Id: I670414cacd970fcc4c21c89689d599f94037c031
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787991
Reviewed-by: Devlin <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Commit-Queue: Evan Stade <[email protected]>
Cr-Commit-Position: refs/heads/master@{#694417}
diff --git a/chrome/browser/extensions/install_tracker.h b/chrome/browser/extensions/install_tracker.h
index 91cdd92..b273963 100644
--- a/chrome/browser/extensions/install_tracker.h
+++ b/chrome/browser/extensions/install_tracker.h
@@ -16,6 +16,7 @@
 #include "components/prefs/pref_change_registrar.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
+#include "extensions/browser/extension_registry.h"
 #include "extensions/browser/extension_registry_observer.h"
 
 namespace content {
@@ -25,7 +26,6 @@
 namespace extensions {
 
 class ExtensionPrefs;
-class ExtensionRegistry;
 
 class InstallTracker : public KeyedService,
                        public content::NotificationObserver,
@@ -92,7 +92,7 @@
   content::NotificationRegistrar registrar_;
   PrefChangeRegistrar pref_change_registrar_;
   ScopedObserver<ExtensionRegistry, ExtensionRegistryObserver>
-      extension_registry_observer_;
+      extension_registry_observer_{this};
 
   DISALLOW_COPY_AND_ASSIGN(InstallTracker);
 };