IWYU: ExtensionRegistry is required in BackgroundContentService

ScopedObserver requires full declaration of the first parameter, it is not
enough to forward declare ExtensionRegistry.

Bug: 819294
Change-Id: Ic053efa2fa70a4117d18613f08f943ddd35b5d5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707363
Commit-Queue: Drew Wilson <atwilson@chromium.org>
Reviewed-by: Drew Wilson <atwilson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678668}

chrome/browser/background/background_contents_service.cc

@@ -41,7 +41,6 @@
 #include "content/public/browser/site_instance.h"
 #include "content/public/browser/web_contents.h"
 #include "extensions/browser/extension_host.h"
-#include "extensions/browser/extension_registry.h"
 #include "extensions/browser/extension_system.h"
 #include "extensions/browser/image_loader.h"
 #include "extensions/browser/notification_types.h"

chrome/browser/background/background_contents_service.h

@@ -20,6 +20,7 @@
 #include "components/keyed_service/core/keyed_service.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"
 #include "net/base/backoff_entry.h"
 #include "ui/base/window_open_disposition.h"
@@ -39,7 +40,6 @@ class SessionStorageNamespace;
 
 namespace extensions {
 class Extension;
-class ExtensionRegistry;
 }  // namespace extensions
 
 namespace gfx {