Also extend GN format presubmit check to .gni files.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3976
Change-Id: I292f3136437101e3e2c3271e260536ab7a24b192
Reviewed-on: https://skia-review.googlesource.com/3976
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
