C++ version of JSON Schema

Way back when we were starting the extensions project, I mentioned that we were using JSON schema for API validation.

This worked out pretty well, but the downside is that it was implemented in JavaScript, so it couldn't really be used effectively in Chrome's browser process (for somewhat obscure technical reasons it's complicated to run JavaScript in Chrome's main process -- we usually only run it in child processes).

So for a really long time, I've been wanting to re-implement it in C++ so that I could use it for work in the browser process too. Finally got around to it:

json_schema_validator.h

This only relies on Chromium's "base" library (which is really quite nice, btw), so it could conceivably be re-used by other projects. For example, I'm hoping that we can use it on the server side, in Chrome's web store.

Figured it might be useful to someone else someday, too.