Fix GN formatting presubmit on Windows.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3182

Change-Id: I6e19b18d6851516357f71930e406b9fb324ea81f
Reviewed-on: https://skia-review.googlesource.com/3182
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

PRESUBMIT.py

@@ -208,7 +208,8 @@ def _CheckGNFormatted(input_api, output_api):
     if not f.LocalPath().endswith('.gn'):
       continue
 
-    cmd = ['gn', 'format', '--dry-run', f.LocalPath()]
+    gn = 'gn.bat' if 'win32' in sys.platform else 'gn'
+    cmd = [gn, 'format', '--dry-run', f.LocalPath()]
     try:
       subprocess.check_output(cmd)
     except subprocess.CalledProcessError: