Split SkSL and utils apart again into .gni files.

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>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index a6d1d16..281ca5a 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -187,7 +187,8 @@
   """Make sure any .gn files we're changing have been formatted."""
   results = []
   for f in input_api.AffectedFiles():
-    if not f.LocalPath().endswith('.gn'):
+    if (not f.LocalPath().endswith('.gn') and
+        not f.LocalPath().endswith('.gni')):
       continue
 
     gn = 'gn.bat' if 'win32' in sys.platform else 'gn'