Add gclient hook to download prebuilt mojo_shell

DEPS

 # This file is used to manage the dependencies of the Chromium src repo. It is
 # used by gclient to determine what version of each dependency to check out, and
 # where.
 #
 # For more information, please refer to the official documentation:
 #   https://sites.google.com/a/chromium.org/dev/developers/how-tos/get-the-code
 #
 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 #
@@ skipping 729 matching lines @@
                 '--no_resume',
                 '--platform=win32',
                 '--directory',
                 '--recursive',
                 '--no_auth',
                 '--num_threads=16',
                 '--bucket', 'chromium-apache-win32',
                 'src/third_party/apache-win32',
     ],
   },
+  # Pull the mojo_shell binary, used for mojo development
+  {
+    'name': 'download_mojo_shell',
+    'pattern': '',
+    'action': [ 'python', 'src/mojo/public/tools/download_shell_binary.py' ],
+  },
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     'pattern': '.',
     'action': ['python', 'src/build/gyp_chromium'],
   },
   {
     # Verify committers' ~/.netc, gclient and git are properly configured for
     # write access to the git repo. To be removed sometime after Chrome to git
     # migration completes (let's say Sep 1 2014).
@@ skipping 10 matching lines @@
     # corresponding .py files have already been deleted.
     'name': 'remove_stale_pyc_files',
     'pattern': 'src/tools/.*\\.py',
     'action': [
         'python',
         'src/tools/remove_stale_pyc_files.py',
         'src/tools',
     ],
   },
 ]