Add some simple HTMLViewer apptests.

mojo/services/html_viewer/public/interfaces/html_viewer.mojom

Index: mojo/services/html_viewer/public/interfaces/html_viewer.mojom
diff --git a/content/test/data/service_worker/fetch_event_blob.js b/mojo/services/html_viewer/public/interfaces/html_viewer.mojom
similarity index 50%
copy from content/test/data/service_worker/fetch_event_blob.js
copy to mojo/services/html_viewer/public/interfaces/html_viewer.mojom
index 8f15e1f1235d0290346920fa79bbebefd9461a1f..100245bf6f60ba6d65f966175c8549105201b696 100644
--- a/content/test/data/service_worker/fetch_event_blob.js
+++ b/mojo/services/html_viewer/public/interfaces/html_viewer.mojom
@@ -2,8 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-this.onfetch = function(event) {
-    var blob = new Blob(['<title>Title</title>']);
-    var response = new Response(blob);
-    event.respondWith(response);
+module mojo;
+
+interface HTMLViewerTestAPI {
+  //WaitForHTMLDocument() => ();
 };
+
+//interface HTMLDocumentTestAPI {
+//  GetLayoutTreeAsText() => (String layout_tree);
+//}