Add a flow class to abstract all the details of minting a token for an app.

chrome/common/net/gaia/oauth2_mint_token_fetcher.h

Index: chrome/common/net/gaia/oauth2_mint_token_fetcher.h
===================================================================
--- chrome/common/net/gaia/oauth2_mint_token_fetcher.h	(revision 124486)
+++ chrome/common/net/gaia/oauth2_mint_token_fetcher.h	(working copy)
@@ -47,10 +47,10 @@
   virtual ~OAuth2MintTokenFetcher();
 
   // Start the flow.
-  void Start(const std::string& oauth_login_access_token,
-             const std::string& client_id,
-             const std::vector<std::string>& scopes,
-             const std::string& origin);
+  virtual void Start(const std::string& oauth_login_access_token,
+                     const std::string& client_id,
+                     const std::vector<std::string>& scopes,
+                     const std::string& origin);
 
   void CancelRequest();