Clean up forward declarations in Source/core

Source/core/streams/UnderlyingSource.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UnderlyingSource_h
 #define UnderlyingSource_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptValue.h"
 #include "platform/heap/Heap.h"
 
 namespace blink {
 
-class ExceptionState;
 class ScriptState;
 
 class UnderlyingSource : public GarbageCollectedMixin {
 public:
     virtual ~UnderlyingSource() { }
 
     virtual void pullSource() = 0;
     virtual ScriptPromise cancelSource(ScriptState*, ScriptValue reason) = 0;
     virtual void trace(Visitor*) { }
 };
 
 } // namespace blink
 
 #endif // UnderlyingSource_h