Unify mc.exe calling in GN, set environment.

remoting/tools/build/message_compiler.gni

-# Copyright 2015 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.
-
-assert(is_win, "This only runs on Windows.")
-
-# Runs mc.exe over a list of sources.
-#
-# sources
-#   List of .mc files to process.
-template("message_compiler") {
-  action_name = "${target_name}_mc"
-  source_set_name = target_name
-
-  action_foreach(action_name) {
-    visibility = [ ":$source_set_name" ]
-    script = "//remoting/tools/build/message_compiler.py"
-
-    sources = invoker.sources
-
-    outputs = [
-      "$target_gen_dir/{{source_name_part}}.h",
-      "$target_gen_dir/{{source_name_part}}.rc",
-    ]
-
-    args = [
-      # Where to put the header.
-      "-h",
-      rebase_path(target_gen_dir, root_build_dir),
-
-      # Where to put the .rc file.
-      "-r",
-      rebase_path(target_gen_dir, root_build_dir),
-
-      # Input is Unicode.
-      "-u",
-      "{{source}}",
-    ]
-
-    if (defined(invoker.deps)) {
-      deps = invoker.deps
-    }
-  }
-
-  source_set(source_set_name) {
-    sources = get_target_outputs(":$action_name")
-    deps = [
-      ":$action_name",
-    ]
-  }
-}