Add gclient hook to download prebuilt mojo_shell

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/url/config.gni b/mojo/public/mojo.gni
similarity index 59%
copy from url/config.gni
copy to mojo/public/mojo.gni
index 42d335fb6228281bd4def9b2d131f0b68e13d618..c0b1a140ab5f115fc2afd7eef0ef582105ec025e 100644
--- a/url/config.gni
+++ b/mojo/public/mojo.gni
@@ -3,6 +3,9 @@
 # found in the LICENSE file.
 
 declare_args() {
-  # Switches to using platform functions instead of ICU on Android.
-  use_icu_alternatives_on_android = false
+  # Whether to use a prebuilt mojo_shell binary instead of one built from
+  # source.
+  use_prebuilt_mojo_shell = false
 }
+
+