Introduce gin_shell

This CL adds a simple shell program for Gin to make edit/test/debug cycle
faster. The shell excutes a list of scripts from the command line and loads any
requested AMD modules relative to the current working directory.

This CL will also let us remove the ugly code in
https://codereview.chromium.org/69843003/diff/240001/mojo/public/bindings/js/test/run_js_tests.cc
because we now know how to file modules via the file system. Eventually for
Mojo, we'll want to use a net_module_provider (instead of the
file_module_provider included in this CL) to load additional AMD modules off
the network.

BUG=317398
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/74753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235750 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235750