Get mojo_shell building inside chromium checkout.

mojo/shell/test/pingable.mojom

Index: mojo/shell/test/pingable.mojom
diff --git a/chrome/browser/resources/pdf/elements/viewer-zoom-selector/viewer-zoom-slider.js b/mojo/shell/test/pingable.mojom
similarity index 58%
copy from chrome/browser/resources/pdf/elements/viewer-zoom-selector/viewer-zoom-slider.js
copy to mojo/shell/test/pingable.mojom
index 767cf846b0d1dca16b86feead3edf91aa2b0181c..74ed38d3d0cd8e820cade5e369318cb4e7ef63a4 100644
--- a/chrome/browser/resources/pdf/elements/viewer-zoom-selector/viewer-zoom-slider.js
+++ b/mojo/shell/test/pingable.mojom
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-Polymer('viewer-zoom-slider', Polymer.mixin({
+module mojo;
 
-}, transitionCenterMixin));
+interface Pingable {
+  Ping(string message) => (string app_url, string connection_url, string message);
+};