[Chromoting] Enable remoting webapp main.html GN build.

remoting/remoting_enable.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.
+
+# Keep in sync with remoting/remoting_enable.gypi.
+
+import("//build/config/ui.gni")
+
+enable_remoting_host = false
+enable_me2me_host = false
+
+if (is_win || is_mac) {
+  enable_remoting_host = true
+  enable_me2me_host = true
+}
+
+if (is_linux && !is_chromeos && use_x11) {
+  enable_remoting_host = true
+  enable_me2me_host = true
+}
+
+if (is_linux && is_chromeos) {
+  enable_remoting_host = true
+}