Support for JSON input conversion

This adds support for reading JSON files in GN and converting them
to equivalent GN values.

Change-Id: I5bca3abb4668da852774d69c98d4dd6575a42957
Reviewed-on: https://chromium-review.googlesource.com/894857
Commit-Queue: Petr Hosek <phosek@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#535078}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c986c354e49a7360b947d3406e8a05ac360b5b1c