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

chrome/browser/extensions/extension_install_prompt.h

Index: chrome/browser/extensions/extension_install_prompt.h
===================================================================
--- chrome/browser/extensions/extension_install_prompt.h	(revision 179928)
+++ chrome/browser/extensions/extension_install_prompt.h	(working copy)
@@ -39,6 +39,7 @@
 class BundleInstaller;
 class Extension;
 class ExtensionWebstorePrivateApiTest;
+class MockGetAuthTokenFunction;
 class PermissionSet;
 }  // namespace extensions
 
@@ -297,8 +298,8 @@
 
  protected:
   friend class extensions::ExtensionWebstorePrivateApiTest;
+  friend class extensions::MockGetAuthTokenFunction;
   friend class WebstoreStandaloneInstallUnpackFailureTest;
-  friend class MockGetAuthTokenFunction;
 
   // Whether or not we should record the oauth2 grant upon successful install.
   bool record_oauth2_grant_;