Make PRESUBMIT.py accept Skia expected results.

To allow submitting Skia/SkiaPaths expected rendering result, make
PRESUBMIT.py accept PNG files with names in the form of
"NAME_expected(_skia)?(_(win|mac|linux))?.pdf.#.png".

Bug: pdfium:1501
Change-Id: I9d2a31baaa6efa32dbd6ccab4121a0f0a1915702
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/67870
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 9ad4abe..b56d75e 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -294,9 +294,9 @@
 def _CheckPNGFormat(input_api, output_api):
   """Checks that .png files have a format that will be considered valid by our
   test runners. If a file ends with .png, then it must be of the form
-  NAME_expected(_(win|mac|linux))?.pdf.#.png"""
+  NAME_expected(_skia)?(_(win|mac|linux))?.pdf.#.png"""
   expected_pattern = input_api.re.compile(
-      r'.+_expected(_(win|mac|linux))?\.pdf\.\d+.png')
+      r'.+_expected(_skia)?(_(win|mac|linux))?\.pdf\.\d+.png')
   results = []
   for f in input_api.AffectedFiles(include_deletes=False):
     if not f.LocalPath().endswith('.png'):