Add gclient hook to download prebuilt mojo_shell

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index e6d019d898de6436394b2ef95b2cccf659f286e3..8703c0dd1d5dc1bfd3f5d7cfdd2c40ed965f029e 100644
--- a/DEPS
+++ b/DEPS
@@ -747,6 +747,12 @@ hooks = [
                 'src/third_party/apache-win32',
     ],
   },
+  # Pull the mojo_shell binary, used for mojo development
+  {
+    'name': 'mojo_shell-linux',
+    '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',