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

mojo/mojo.gyp

 # Copyright 2013 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.
 
 {
   'target_defaults': {
     'conditions': [
       ['mojo_shell_debug_url != ""', {
         'defines': [
           'MOJO_SHELL_DEBUG=1',
@@ skipping 538 matching lines @@
           ],
           'defines': [
             'MOJO_VIEWS_IMPLEMENTATION',
           ],
         },
       ],
     }],
     ['component!="shared_library" and OS=="linux"', {
       'targets': [
         {
+          # GN version: //mojo/public/python:system
           'target_name': 'mojo_python_system',
           'variables': {
             'python_base_module': 'mojo',
             'python_cython_module': 'system',
           },
           'sources': [
             'public/python/mojo/c_core.pxd',
             'public/python/mojo/system.pyx',
           ],
           'dependencies': [
             'mojo_base.gyp:mojo_system',
           ],
           'includes': [ '../third_party/cython/cython_compiler.gypi' ],
         },
         {
+          # GN version: //mojo/python:embedder
           'target_name': 'mojo_python_embedder',
           'type': 'loadable_module',
           'variables': {
             'python_base_module': 'mojo',
             'python_cython_module': 'embedder',
           },
           'sources': [
             'python/system/mojo/embedder.pyx',
           ],
           'dependencies': [
             'mojo_base.gyp:mojo_system_impl',
           ],
           'includes': [ '../third_party/cython/cython_compiler.gypi' ],
         },
         {
+          # GN version: //mojo/public/python:bindings
           'target_name': 'mojo_python_bindings',
           'type': 'none',
           'variables': {
             'python_base_module': 'mojo/bindings',
           },
           'sources': [
             'public/python/mojo/bindings/__init__.py',
             'public/python/mojo/bindings/descriptor.py',
             'public/python/mojo/bindings/reflection.py',
           ],
           'dependencies': [
             'mojo_python_system',
           ],
           'includes': [ '../third_party/cython/python_module.gypi' ],
         },
         {
+          # GN version: //mojo/python
           'target_name': 'mojo_python',
           'type': 'none',
           'variables': {
             'python_base_module': 'mojo',
           },
           'sources': [
             'public/python/mojo/__init__.py',
           ],
           'dependencies': [
             'mojo_python_bindings',
             'mojo_python_embedder',
             'mojo_python_system',
           ],
           'includes': [ '../third_party/cython/python_module.gypi' ],
         },
       ],
     }],
   ],
 }