Add a "gn path" command for finding deps.

tools/gn/BUILD.gn

 # 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.
 
 import("//testing/test.gni")
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   sources = [
@@ skipping 14 matching lines @@
     "c_include_iterator.cc",
     "c_include_iterator.h",
     "command_args.cc",
     "command_check.cc",
     "command_clean.cc",
     "command_desc.cc",
     "command_format.cc",
     "command_gen.cc",
     "command_help.cc",
     "command_ls.cc",
+    "command_path.cc",
     "command_refs.cc",
     "commands.cc",
     "commands.h",
     "config.cc",
     "config.h",
     "config_values.cc",
     "config_values.h",
     "config_values_extractors.cc",
     "config_values_extractors.h",
     "config_values_generator.cc",
@@ skipping 241 matching lines @@
 }
 
 executable("generate_test_gn_data") {
   sources = [
     "generate_test_gn_data.cc",
   ]
   deps = [
     "//base",
   ]
 }