Add a function triggering extension installed to ExtensionRegistryObserver

extensions/browser/extension_registry_observer.h

Index: extensions/browser/extension_registry_observer.h
diff --git a/extensions/browser/extension_registry_observer.h b/extensions/browser/extension_registry_observer.h
index 5880add25e63307af4691dae2e4bc98bdbcd7b99..31d6ad88e1d388a846cfc2d61b7ef01598bfde08 100644
--- a/extensions/browser/extension_registry_observer.h
+++ b/extensions/browser/extension_registry_observer.h
@@ -33,6 +33,13 @@ class ExtensionRegistryObserver {
   virtual void OnExtensionUnloaded(content::BrowserContext* browser_context,
                                    const Extension* extension,
                                    UnloadedExtensionInfo::Reason reason) {}
+
+  // Called after an extension is installed. The extension will be in exactly
+  // one of the ExtensionRegistry sets.

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

This comment which I asked you to write is actually incorrect, right? The
extension will be in NONE of the ExtensionRegistry sets (based on your DCHECK in
TriggerOnInstalled and reading the code in ExtensionService).

Like I mentioned there, that's a really weird contract, particularly given the
name.

The best idea I've come up with so far (like I mentioned) is to call this
OnExtensionWillBeInstalled, and in the comment emphasize
- the extension won't be in the registry
- the extension is about to appear in the registry (but could be in any of the
sets)
- callers probably want to listen for OnLoaded unless they really do want to
know when an extension is about to be installed.

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

Yep, ExtensionService send notification prior to actual end of install.
(from here, the notification of NOTIFICATION_EXTENSION_INSTALLED is also
incorrect, isn't it? I think it also should be 
NOTIFICATION_EXTENSION_WILLBEINSTALLED)

[not at google - send to devlin, 2014/05/16 23:03:35]

yes exactly. I expect at least one person has been confused by this over the
years...

+  virtual void OnExtensionInstalled(content::BrowserContext* browser_context,
+                                    const Extension* extension,
+                                    bool is_update,
+                                    const std::string& old_name) {}
 };
 
 }  // namespace extensions