Teach mojo_js_bindings_unittests to load generated JS files

This CL generalizes gin::FileModuleProvider to be able to load modules from
several places in the file system. I've then made use of this facility in
mojo_js_bindings_unittests to load generated JavaScript modules created by
mojom_js_generator.py, similar to how C++ files can #include from the generated
mojom directory.

I've deleted sample_service.js now that we can load it from the generated
directory.

BUG=317398

Review URL: https://codereview.chromium.org/81673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236711 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed