build_gn.py: Forward BUNDLED_WIN_TOOLCHAIN_ROOT

And tweak --depot-tools to go first on PATH

Change-Id: Iee53d84fd028ac0c2de6f872184cbce51e84c54f
Reviewed-on: https://chromium-review.googlesource.com/1028210Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52788}

tools/node/build_gn.py

@@ -103,6 +103,8 @@ def ParseOptions(args):
   parser.add_argument("--host_os", help="Current operating system")
   parser.add_argument("--bundled-win-toolchain",
                       help="Value for DEPOT_TOOLS_WIN_TOOLCHAIN")
+  parser.add_argument("--bundled-win-toolchain-root",
+                      help="Value for DEPOT_TOOLS_WIN_TOOLCHAIN_ROOT")
   parser.add_argument("--depot-tools", help="Absolute path to depot_tools")
   parser.add_argument("--extra-gn-args", help="Additional GN args")
   parser.add_argument("--build", help="Run ninja as opposed to gn gen.",
@@ -127,8 +129,14 @@ if __name__ == "__main__":
   options = ParseOptions(sys.argv[1:])
   # Build can result in running gn gen, so need to set environment variables
   # for build as well as generate.
-  os.environ['DEPOT_TOOLS_WIN_TOOLCHAIN'] = options.bundled_win_toolchain
-  os.environ['PATH'] += os.path.pathsep + options.depot_tools
+  if options.bundled_win_toolchain:
+    os.environ['DEPOT_TOOLS_WIN_TOOLCHAIN'] = options.bundled_win_toolchain
+  if options.bundled_win_toolchain_root:
+    os.environ['DEPOT_TOOLS_WIN_TOOLCHAIN_ROOT'] = (
+        options.bundled_win_toolchain_root)
+  if options.depot_tools:
+    os.environ['PATH'] = (
+        options.depot_tools + os.path.pathsep + os.environ['PATH'])
   if not options.build:
     GenerateBuildFiles(options)
   else: