Add gclient hook to download prebuilt mojo_shell

mojo/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/ui.gni")
+import("//mojo/public/mojo.gni")
 
 group("mojo") {
   # Meta-target, don't link into production code.
   testonly = true
   declare_args() {
     mojo_use_go = false
   }
   deps = [
     ":tests",
     "//mojo/apps/js:mojo_js_content_handler",
     "//mojo/apps/js:mojo_js_standalone",
     "//mojo/common",
     "//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",
     ]
   }
 
   if (is_linux) {
     deps += [
       "//mojo/python",
     ]
@@ skipping 43 matching lines @@
     sources = [
       "android/javatests/src/org/chromium/mojo/MojoTestCase.java",
       "android/javatests/src/org/chromium/mojo/bindings/ValidationTestUtil.java",
       "android/system/src/org/chromium/mojo/system/impl/CoreImpl.java",
       "services/native_viewport/android/src/org/chromium/mojo/PlatformViewportAndroid.java",
     ]
 
     jni_package = "mojo"
   }
 }