GN: add extra_cflags et al.

infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Fast.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_/_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 105 matching lines @@
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Fast"
     },
     "name": "fetch-gn"
   },
   {
     "cmd": [
       "gn",
       "gen",
       "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Fast/Release",
-      "--args=is_debug=false cc=\"gcc -march=native -fomit-frame-pointer\" cxx=\"g++ -march=native -fomit-frame-pointer\""
+      "--args=cc=\"gcc\" cxx=\"g++\" is_debug=false extra_cflags=\"-march=native -fomit-frame-pointer -O3\""
     ],
     "cwd": "[CUSTOM_/_B_WORK]/skia",
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "GYP_DEFINES": "skia_arch_type=x86_64 skia_fast=1 skia_warnings_as_errors=0",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Fast"
     },
     "name": "gn gen"
@@ skipping 50 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    shutil.move(f, dst_path)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]