This ports the build_config json from GN to GYP to prepare deprecating
tedious flags passing to the test runner.

This also removes two unused GN flags that only hold temporary values.

Bug: v8:6917
Change-Id: I976185f1541277dc5c9bfbaa7578f35c19dd254c
Reviewed-on: https://chromium-review.googlesource.com/725706
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48715}