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.cc b/chrome/browser/extensions/install_tracker.cc
index fbffefc3..abcf8e9 100644
--- a/chrome/browser/extensions/install_tracker.cc
+++ b/chrome/browser/extensions/install_tracker.cc
@@ -9,15 +9,13 @@
 #include "chrome/browser/extensions/install_tracker_factory.h"
 #include "content/public/browser/notification_service.h"
 #include "extensions/browser/extension_prefs.h"
-#include "extensions/browser/extension_registry.h"
 #include "extensions/browser/extension_system.h"
 #include "extensions/browser/pref_names.h"
 
 namespace extensions {
 
 InstallTracker::InstallTracker(content::BrowserContext* browser_context,
-                               extensions::ExtensionPrefs* prefs)
-    : extension_registry_observer_(this) {
+                               extensions::ExtensionPrefs* prefs) {
   registrar_.Add(this,
                  extensions::NOTIFICATION_EXTENSION_UPDATE_DISABLED,
                  content::Source<content::BrowserContext>(browser_context));