This makes integration with Chrome much easier: Chrome won't
have to pull sources from github or an internal mirror, and
it won't duplicate the base json library.

This should also fix warnings about missing headers with
building Chromium gyp with the ninja msvs generator.

BUG=459058

Change-Id: Ie97cfde9b848a9099da696e027887bcc07b55e08
Reviewed-on: https://chromium-review.googlesource.com/252073Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>