clang/blink_gc_plugin: Replace 'WebCore' with 'blink'

tools/clang/blink_gc_plugin/tests/weak_fields_require_tracing.cpp

 // 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.
 
 #include "weak_fields_require_tracing.h"
 
-namespace WebCore {
+namespace blink {
 
 void HeapObject::trace(Visitor* visitor)
 {
     // Missing visitor->trace(m_obj1);
     // Missing visitor->trace(m_obj2);
     // visitor->trace(m_obj3) in callback.
     // Missing visitor->trace(m_set1);
     visitor->trace(m_set2);
     visitor->registerWeakMembers<HeapObject,
                                  &HeapObject::clearWeakMembers>(this);
 }
 
 void HeapObject::clearWeakMembers(Visitor* visitor)
 {
     visitor->trace(m_obj1);  // Does not count.
     // Missing visitor->trace(m_obj2);
     visitor->trace(m_obj3);  // OK.
     visitor->trace(m_set1);  // Does not count.
 }
 
 }