Add a function triggering extension installed to ExtensionRegistryObserver

extensions/browser/extension_registry.h

Index: extensions/browser/extension_registry.h
diff --git a/extensions/browser/extension_registry.h b/extensions/browser/extension_registry.h
index d8df24b3653d51b30a7b5996ae96579442d832a9..3a2b49bc752caac53488e3c20aec75f9f7983cb6 100644
--- a/extensions/browser/extension_registry.h
+++ b/extensions/browser/extension_registry.h
@@ -74,6 +74,12 @@ class ExtensionRegistry : public KeyedService {
   void TriggerOnUnloaded(const Extension* extension,
                          UnloadedExtensionInfo::Reason reason);
 
+  // Invokes the observer method OnExtensionInstalled(). The extension must not
+  // be installed at the time of the call.

[not at google - send to devlin, 2014/05/16 13:49:03]

Ugh. I think the notification is badly named here, because "extension must not
be installed" is exactly the opposite behaviour to what I'd imagine.

How about we name this "will be installed"?

[limasdf, 2014/05/16 20:39:14]

DONE. the original sentance is also correct. because of
DCHECK(!GenerateInstalledExtensionsSet()->Contains(extension->id())); from the
body of TriggerOnInstalled.
the notification indicates the DCHECK condition, doesn't it?

+  void TriggerOnInstalled(const Extension* extension,
+                          bool is_update,
+                          const std::string& old_name);
+
   // Find an extension by ID using |include_mask| to pick the sets to search:
   //  * enabled_extensions()     --> ExtensionRegistry::ENABLED
   //  * disabled_extensions()    --> ExtensionRegistry::DISABLED