Migrate to using gin bindings

[Chris Coulson]

We have some hacky code to expose certain objects on the renderer side to V8 (ScriptMessage, ScriptMessageRequest etc), but we should get rid of this and migrate to using gin bindings instead. See https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/JwSMYicKECQ.

It looks like we just need C++ objects to inherit from gin::Wrappable, and they should implement GetObjectTemplateBuilder(). gin::Handle can be used to retain a gin::Wrappable on the stack, but it doesn't look like gin provides a way to retain one from the heap. We could probably do this easily enough by providing a helper class which takes a gin::Wrappable and holds its wrapper in a v8::Persistent