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

tools/clang/blink_gc_plugin/tests/base_requires_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 "base_requires_tracing.h"
 
-namespace WebCore {
+namespace blink {
 
 void A::trace(Visitor* visitor) { }
 
 void C::trace(Visitor* visitor) {
   visitor->trace(m_a);
   // Missing B::trace(visitor)
 }
 
 void D::trace(Visitor* visitor) {
   visitor->trace(m_a);
   C::trace(visitor);
 }
 
 }