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/extension_browser_window_helper.h b/chrome/browser/extensions/extension_browser_window_helper.h
index 3ba0c49..803e62c 100644
--- a/chrome/browser/extensions/extension_browser_window_helper.h
+++ b/chrome/browser/extensions/extension_browser_window_helper.h
@@ -7,6 +7,7 @@
 
 #include "base/macros.h"
 #include "base/scoped_observer.h"
+#include "extensions/browser/extension_registry.h"
 #include "extensions/browser/extension_registry_observer.h"
 
 class Browser;
@@ -36,7 +37,7 @@
   Browser* const browser_ = nullptr;
 
   ScopedObserver<ExtensionRegistry, ExtensionRegistryObserver>
-      registry_observer_;
+      registry_observer_{this};
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionBrowserWindowHelper);
 };