Standalone Mojo Javascript application

Refactored the JSApp et al classes to enable launching apps with the content launcher (as before) as well as launching a single standalone JS app.

The content handler class has been moved to its own file. The part of the JSApp Start() method that loads the initial script is now an abstract method called Load(). There are content handler and standalone versions of JSApp::Load().

The code that's specific to the standalone case is now in standalone_main.cc. The code that's specific to the content handler case is in content_handler_main.cc and content_handler_impl.{h,cc}.

Currently the standalone app's filename is wired into standalone_main.cc.  Once crbug.com/418797 has been fixed the filename can be passed through mojo_shell.

BUG=417530

Review URL: https://codereview.chromium.org/608333002

Cr-Commit-Position: refs/heads/master@{#297675}