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

tools/clang/blink_gc_plugin/tests/garbage_collected_mixin.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 "garbage_collected_mixin.h"
 
-namespace WebCore {
+namespace blink {
 
 void Mixin::trace(Visitor* visitor)
 {
     // Missing: visitor->trace(m_self);
 }
 
 void HeapObject::trace(Visitor* visitor)
 {
     visitor->trace(m_mix);
     // Missing: Mixin::trace(visitor);
 }
 
 }