mojo: Update gyp python targets to specify their equivalent gn targets.

mojo/python/BUILD.gn

 # Copyright 2014 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.
 
 import("//third_party/cython/rules.gni")
 
+# GYP version: mojo/mojo.gyp:mojo_python
 group("python") {
   deps = [
     ":embedder",
+    "//mojo/public/python",
   ]
 }
 
+# GYP version: mojo/mojo.gyp:mojo_python_embedder
 python_binary_module("embedder") {
   python_base_module = "mojo"
   sources = [
     "system/mojo/embedder.pyx",
   ]
   deps = [
     "//mojo/system",
   ]
   datadeps = [
     "//mojo/public/python:system",
   ]
 }