GN: expunge all environment variables that have no effect on GN

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 62 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_revision@164710@@@"
     ]
   },
   {
     "cmd": [
       "[CUSTOM_C:\\_B_WORK]\\skia\\bin\\fetch-gn"
     ],
     "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"
+      "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "fetch-gn"
   },
   {
     "cmd": [
       "gn",
       "gen",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN\\Release",
       "--args=cc=\"cc\" compiler_prefix=\"\" cxx=\"c++\" extra_cflags=\"\" is_debug=false"
     ],
     "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"
+      "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gn gen"
   },
   {
     "cmd": [
       "ninja",
       "-C",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN\\Release"
     ],
     "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"
+      "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "ninja"
   },
   {
     "cmd": [
       "python",
       "-u",
       "import errno\nimport glob\nimport os\nimport shutil\nimport sys\n\nsrc = sys.argv[1]\ndst = sys.argv[2]\nbuild_products_whitelist = ['dm', 'dm.exe', 'get_images_from_skps', 'get_images_from_skps.exe', 'nanobench', 'nanobench.exe', '*.so', '*.dll', '*.dylib', 'skia_launcher', 'lib/*.so', 'iOSShell.app', 'iOSShell.ipa', 'visualbench', 'visualbench.exe', 'vulkan-1.dll']\n\ntry:\n  os.makedirs(dst)\nexcept OSError as e:\n  if e.errno != errno.EEXIST:\n    raise\n\nfor pattern in build_products_whitelist:\n  path = os.path.join(src, pattern)\n  for f in glob.glob(path):\n    dst_path = os.path.join(dst, os.path.relpath(f, src))\n    if not os.path.isdir(os.path.dirname(dst_path)):\n      os.makedirs(os.path.dirname(dst_path))\n    print 'Copying build product %s to %s' % (f, dst_path)\n    shutil.move(f, dst_path)\n",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN\\Release",
       "[CUSTOM_[SWARM_OUT_DIR]]\\out\\Release"
@@ skipping 44 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    pass@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]