Create native_viewport_service, don't build it into mojo_shell unless Android

mojo/services/native_viewport/BUILD.gn

Index: mojo/services/native_viewport/BUILD.gn
diff --git a/mojo/services/native_viewport/BUILD.gn b/mojo/services/native_viewport/BUILD.gn
index 34ec95f8ee8f9f1a52c4b7214a982c71f40f13ec..83afa7c0211789a7bd0108afcd9dbb93d330d82f 100644
--- a/mojo/services/native_viewport/BUILD.gn
+++ b/mojo/services/native_viewport/BUILD.gn
@@ -5,10 +5,20 @@
 import("//build/config/ui.gni")
 import("//mojo/system.gni")
 
-static_library("native_viewport") {
+shared_library("native_viewport") {
   output_name = "mojo_native_viewport"
 
   deps = [
+    ":lib",
+    "//mojo/services/public/interfaces/native_viewport",
+  ]
+  deps += mojo_system_for_shared_library
+
+  sources = [ "main.cc" ]
+}
+
+source_set("lib") {
+  deps = [
     "//base",
     "//cc/surfaces",
     "//mojo/common",
@@ -49,13 +59,6 @@ static_library("native_viewport") {
     deps += [ "//mojo:jni_headers" ]
   }
 
-  if (is_win) {
-    deps += [
-      # TODO(GYP)
-      # '../ui/platform_window/win/win_window.gyp:win_window',
-    ]
-  }
-
   if (use_x11) {
     sources += [ "platform_viewport_x11.cc" ]
     deps += [