Make CustomElementSchedular static

Source/core/dom/custom/CustomElementProcessingStack.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 45 matching lines @@
                 processElementQueueAndPop();
             s_elementQueueStart = m_savedElementQueueStart;
         }
 
     private:
         size_t m_savedElementQueueStart;
     };
 
     static bool inCallbackDeliveryScope() { return s_elementQueueStart; }
 
-protected:
-    friend class CustomElementScheduler;
     static CustomElementProcessingStack& instance();
     void enqueue(CustomElementCallbackQueue*);
 
 private:
     CustomElementProcessingStack()
     {
         // Add a null element as a sentinel. This makes it possible to
         // identify elements queued when there is no
         // CallbackDeliveryScope active. Also, if the processing stack
         // is popped when empty, this sentinel will cause a null deref
@@ skipping 20 matching lines @@
     // The processing stack, flattened. Element queues lower in the
     // stack appear toward the head of the vector. The first element
     // is a null sentinel value.
     static const size_t kNumSentinels = 1;
     Vector<CustomElementCallbackQueue*> m_flattenedProcessingStack;
 };
 
 }
 
 #endif // CustomElementProcessingStack_h