app_shell: Extract extension runtime data from ExtensionService

extensions/browser/extension_registry_observer.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
+#define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
+
+namespace extensions {
+
+class Extension;
+
+// Observer for ExtensionRegistry. Exists in a separate header file to reduce
+// the include file burden for typical clients of ExtensionRegistry.
+class ExtensionRegistryObserver {
+ public:
+  virtual ~ExtensionRegistryObserver() {}
+
+  // Called after an extension is unloaded. The extension no longer exists in
+  // any of the ExtensionRegistry sets (enabled, disabled, etc.).
+  virtual void OnExtensionUnloaded(const Extension* extension) = 0;

[not at google - send to devlin, 2014/01/22 16:17:43]

I can imagine this interface getting a few times bigger, once all of the
extension lifetime events are also present here. Point being: should they be

virtual void OED(e) {};

rather than

virtual void OED(e) = 0;

? Chances are classes only care about unloading anyway.

[James Cook, 2014/01/22 17:41:52]

Sure, I'll do that.  I've seen both patterns for interfaces in Chrome.

+
+  // Called after an extension is disabled. The extension exists in the
+  // ExtensionRegistry disabled set when this function is called.
+  virtual void OnExtensionDisabled(const Extension* extension) = 0;

[not at google - send to devlin, 2014/01/22 16:17:43]

see comment in ExtensionRegistry about a giant warning :)

+};
+
+}
+
+#endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_