Cleanup remaining class/struct fwd declarations

tools/gn/header_checker.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 TOOLS_GN_HEADER_CHECKER_H_
 #define TOOLS_GN_HEADER_CHECKER_H_
 
 #include <map>
 #include <set>
 #include <vector>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/run_loop.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "tools/gn/err.h"
 
 class BuildSettings;
 class InputFile;
-class Label;
 class LocationRange;
 class SourceFile;
 class Target;
 
 namespace base {
 class MessageLoop;
 }
 
 class HeaderChecker : public base::RefCountedThreadSafe<HeaderChecker> {
  public:
@@ skipping 141 matching lines @@
   // These are mutable during runtime and require locking.
 
   base::Lock lock_;
 
   std::vector<Err> errors_;
 
   DISALLOW_COPY_AND_ASSIGN(HeaderChecker);
 };
 
 #endif  // TOOLS_GN_HEADER_CHECKER_H_