Add remaining remoting targets for the GN build.

remoting/remoting_host.gni

Index: remoting/remoting_host.gni
diff --git a/content/browser/background_sync/BUILD.gn b/remoting/remoting_host.gni
similarity index 50%
copy from content/browser/background_sync/BUILD.gn
copy to remoting/remoting_host.gni
index 55aeded7a53a4379bfa84619a1796410c1e7a0fa..a764bf2847bc2287b04cc7ead863ffe86530ef71 100644
--- a/content/browser/background_sync/BUILD.gn
+++ b/remoting/remoting_host.gni
@@ -2,10 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//third_party/protobuf/proto_library.gni")
+import("//build/config/ui.gni")
 
-proto_library("background_sync_proto") {
-  sources = [
-    "background_sync.proto",
-  ]
-}
+enable_remoting_host = is_win || is_mac || is_chromeos || use_x11
+enable_me2me_host = is_win || is_mac || (use_x11 && !is_chromeos)