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

mojo/public/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")
 
 group("python") {
   deps = [
     ":base",
     ":bindings",
     ":system",
   ]
 }
 
+# GYP version: mojo.gyp:mojo_python_system
 python_binary_module("system") {
   python_base_module = "mojo"
   sources = [
     "mojo/c_core.pxd",
     "mojo/system.pyx",
   ]
   deps = [
     "//mojo/public/c/system",
     ":base",
   ]
 }
 
 copy("base") {
   sources = [
     "mojo/__init__.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/{{source_file_part}}",
   ]
 }
 
+# GYP version: mojo.gyp:mojo_python_bindings
 copy("bindings") {
   sources = [
     "mojo/bindings/__init__.py",
     "mojo/bindings/descriptor.py",
     "mojo/bindings/reflection.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/bindings/{{source_file_part}}",
   ]
   deps = [
     ":base",
     ":system",
   ]
 }