Update mojo.js to use native_viewport

This CL moves mojo.js from using hello_world_service to using native_viewport.
Now mojo.js creates a viewport, but we're still not able to draw GL becaues we
don't have WebGL bindings.

R=aa@chromium.org
BUG=324643

Review URL: https://codereview.chromium.org/98423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238834 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238834