[NaCl SDK] Add rotating cube App Engine demo.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/54393005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232465 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232465