Get rid of MessageLoop* caching in extensions code.

chrome/browser/chrome_thread.h

Index: chrome/browser/chrome_thread.h
===================================================================
--- chrome/browser/chrome_thread.h	(revision 30521)
+++ chrome/browser/chrome_thread.h	(working copy)
@@ -121,14 +121,14 @@
   //   unless you've specifically spun up the threads, but be mindful of it.
   static bool CurrentlyOn(ID identifier);
 
+  // If the current message loop is one of the known threads, returns true and
+  // sets identifier to its ID.  Otherwise returns false.
+  static bool GetCurrentThreadIdentifier(ID* identifier);
+
  private:
   // Common initialization code for the constructors.
   void Initialize();
 
-  // If the current message loop is one of the known threads, returns true and
-  // sets identifier to its ID.  Otherwise returns false.
-  static bool GetCurrentThreadIdentifier(ID* identifier);
-
   static bool PostTaskHelper(
       ID identifier,
       const tracked_objects::Location& from_here,