tools/gn: forward declare some base types in trace.h

tools/gn/trace.h

 // Copyright (c) 2013 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_TRACE_H_
 #define TOOLS_GN_TRACE_H_
 
 #include <string>
 
-#include "base/command_line.h"
-#include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/threading/platform_thread.h"
 #include "base/time/time.h"
 
 class Label;
 
+namespace base {
+class CommandLine;
+class FilePath;
+}
+
 class TraceItem {
  public:
   enum Type {
     TRACE_SETUP,
     TRACE_FILE_LOAD,
     TRACE_FILE_PARSE,
     TRACE_FILE_EXECUTE,
     TRACE_FILE_WRITE,
     TRACE_SCRIPT_EXECUTE,
     TRACE_DEFINE_TARGET,
@@ skipping 61 matching lines @@
 void AddTrace(TraceItem* item);
 
 // Returns a summary of the current traces, or the empty string if tracing is
 // not enabled.
 std::string SummarizeTraces();
 
 // Saves the current traces to the given filename in JSON format.
 void SaveTraces(const base::FilePath& file_name);
 
 #endif  // TOOLS_GN_TRACE_H_