Implement the rest of the PPAPI examples in the GN build.

ppapi/examples/gles2_spinning_cube/BUILD.gn

 # Copyright 2015 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("//ppapi/examples/ppapi_example.gni")
 
-ppapi_example("audio") {
-  output_name = "ppapi_example_audio"
+ppapi_example("gles2_spinning_cube") {
+  output_name = "ppapi_example_gles2_spinning_cube"
   sources = [
-    "audio.cc",
+    "gles2_spinning_cube.cc",
+    "spinning_cube.cc",
+    "spinning_cube.h",
   ]
   deps = [
     "//ppapi/cpp",
+    "//ppapi/lib/gl/gles2",
   ]
 }