Move "oauth2" manifest key parsing out of Extension class.

chrome/common/extensions/manifest_handler.h

Index: chrome/common/extensions/manifest_handler.h
===================================================================
--- chrome/common/extensions/manifest_handler.h	(revision 176209)
+++ chrome/common/extensions/manifest_handler.h	(working copy)
@@ -30,6 +30,12 @@
                      Extension* extension,
                      string16* error) = 0;
 
+  // Perform any initialization which is necessary when the Handler's key is
+  // not present in the manifest.
+  // Returns true on success or false on failure; if false, |error| will
+  // be set to a failure message.
+  virtual bool HasNoKey(Extension* extension, string16* error);
+
   // Associate |handler| with |key| in the manifest. Takes ownership
   // of |handler|. TODO(yoz): Decide how to handle dotted subkeys.
   // WARNING: Manifest handlers registered only in the browser process