[Sync] Add support for enabling/disabling an extension before it's installed

chrome/browser/extensions/extension_service.h

Index: chrome/browser/extensions/extension_service.h
diff --git a/chrome/browser/extensions/extension_service.h b/chrome/browser/extensions/extension_service.h
index 94f15a94df24789b9f7ecadd17bf0dbb2f2ec2ba..a18aab50c7cf52314ffcaa06a4faa5bb86f86652 100644
--- a/chrome/browser/extensions/extension_service.h
+++ b/chrome/browser/extensions/extension_service.h
@@ -276,9 +276,12 @@ class ExtensionService
   virtual bool IsExternalExtensionUninstalled(
       const std::string& extension_id) const OVERRIDE;
 
-  // Enable or disable an extension. No action if the extension is already
-  // enabled/disabled.
+  // Enables the extension.  If the extension is already enabled, does
+  // nothing.
   virtual void EnableExtension(const std::string& extension_id);
+
+  // Disables the extension.  If the extension is already disabled, or
+  // cannot be disabled, does nothing.
   virtual void DisableExtension(const std::string& extension_id);
 
   // Updates the |extension|'s granted permissions lists to include all