Move back to using Chrome on OSX closes #2043

This moves back to using Chrome on OSX but making it obvious that it's Chrome to avoid any user issues with running Chrome itself. This also includes a more general refactoring of the browser launching code, as it had gotten a little convoluted after various revisions. This should be a bit more maintainable and portable.
All I'm suggesting is moving the code that adds the -app flag into addDefaultChromeArgs and adding a boolean parameter to control whether or not that's added.  That would just more clearly express that this is something that's usually done, except on OS X and would provide something to which to anchor your comment.  You can still keep other os-specific stuff in the respective classes.

Really not a big deal, just a minor suggestion and nothing that gates the PR.