Add gclient hook to download prebuilt mojo_shell

mojo/BUILD.gn

Index: mojo/BUILD.gn
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index 8f58fc1487c8c95c846c7d839f1bee3c29a42c0d..7b188e70e46c8ca077cf80053334a5a99d821d62 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//mojo/public/mojo.gni")
 
 group("mojo") {
   # Meta-target, don't link into production code.
@@ -18,10 +19,15 @@ group("mojo") {
     "//mojo/examples",
     "//mojo/public",
     "//mojo/services",
-    "//mojo/shell:mojo_shell",
     "//mojo/tools/package_manager",
   ]
 
+  if (use_prebuilt_mojo_shell) {
+    deps += [ "//mojo/public/tools:copy_mojo_shell" ]
+  } else {
+    deps += [ "//mojo/shell:mojo_shell" ]
+  }
+
   if (is_android) {
     deps += [
       "//mojo/android",