Windows support

I'd like to get libchromiumcontent building on Windows. This PR tracks that work.

### To do

* [x] Figure out how to combine all the static libraries into a single shared library. Right now we're building an essentially empty DLL.
* [x] Reenable building of `test_support_chromiumcontent`. (#13)
* [x] Figure out what binaries we need to include in the distribution.
* [x] Make `script/bootstrap` install Windows dependencies.
* [x] Only copy files into `vendor/chromium/src/chromiumcontent` if they've changed
* [x] See if we can remove some of the hacks around which libs/symbols we export in `gyp-win-tool-wrapper`
* [x] Upload symbols to S3