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

CommandLine and FilePath are being passed by reference, so they can be
forward declared and their header includes be moved to the source file
instead.

BUG=None
TEST=gn gen + gn_unittests
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1492813002

Cr-Commit-Position: refs/heads/master@{#363056}

tools/gn/trace.cc

@@ -9,6 +9,8 @@
 #include <sstream>
 #include <vector>
 
+#include "base/command_line.h"
+#include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/json/string_escape.h"
 #include "base/logging.h"

tools/gn/trace.h

@@ -7,14 +7,17 @@
 
 #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 {