Teach mojo_shell to load apps from the network

This CL changes mojo_shell from taking the app as a local file path to taking a
URL to the app.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/32363006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230074 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230074