Add a function triggering extension installed to ExtensionRegistryObserver

extensions/browser/extension_registry.cc

Index: extensions/browser/extension_registry.cc
diff --git a/extensions/browser/extension_registry.cc b/extensions/browser/extension_registry.cc
index ac6c274db6c13cbe21986b4f98ddc69165bda2b4..3b71b0384ac67ab80badd4119523419e8cf986ee 100644
--- a/extensions/browser/extension_registry.cc
+++ b/extensions/browser/extension_registry.cc
@@ -53,6 +53,15 @@ void ExtensionRegistry::TriggerOnUnloaded(
                     OnExtensionUnloaded(browser_context_, extension, reason));
 }
 
+void ExtensionRegistry::TriggerOnWillBeInstalled(const Extension* extension,
+                                                 bool is_update,
+                                                 const std::string& old_name) {

[not at google - send to devlin, 2014/05/20 18:27:12]

I think the DCHECK actually was right and the unit test was wrong. If it's an
update then a new extension should have been generated.

But the DCHECK was wrong in that there should be an extension with that *id*
already installed, depending on the value of |is_update|.

[limasdf, 2014/05/21 15:35:08]

Done. yeah exactly should do DCHECK depending on |is_update|.

+  FOR_EACH_OBSERVER(ExtensionRegistryObserver,
+                    observers_,
+                    OnExtensionWillBeInstalled(
+                        browser_context_, extension, is_update, old_name));
+}
+
 const Extension* ExtensionRegistry::GetExtensionById(const std::string& id,
                                                      int include_mask) const {
   std::string lowercase_id = StringToLowerASCII(id);