Fold extra_*_flags into GN as a config.

This should make them visible to things like gn_to_cmake.py.
I'm not exactly sure what this implies about ordering and overriding.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2938

Change-Id: I0740613993fb5bbfb8363cfa126d1f59768abf60
Reviewed-on: https://skia-review.googlesource.com/2938
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
8 files changed