GN: add extra_cflags et al.

infra/bots/recipes/swarm_compile.expected/Build-Win-MSVC-x86-Release-GN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[CUSTOM_C:\\_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 75 matching lines @@
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN"
     },
     "name": "fetch-gn"
   },
   {
     "cmd": [
       "gn",
       "gen",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN\\Release",
-      "--args=is_debug=false cc=\"cc \" cxx=\"c++ \""
+      "--args=cc=\"cc\" cxx=\"c++\" is_debug=false extra_cflags=\"\""
     ],
     "cwd": "[CUSTOM_C:\\_B_WORK]\\skia",
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "GYP_DEFINES": "qt_sdk=C:/Qt/4.8.5/ skia_arch_type=x86 skia_warnings_as_errors=1 skia_win_debuggers_path=c:/DbgHelp skia_win_ltcg=0",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN"
     },
     "name": "gn gen"
@@ skipping 68 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    pass@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]