Update pdfium_test revision and add a note in PRESUBMIT.py.

In pdfium_test, the expected .png file name used for presubmit check has
been updated in
https://pdfium-review.googlesource.com/c/pdfium_tests/+/85572.

This CL updates the pdfium_test revision to roll this change and adds a
note to keep the expected .png file name format consistent for presubmit
checks in PDFium and pdfium_test.

Bug: pdfium:1727
Change-Id: I6745f30b5b1a6f65bc73db89f37ab46ff787c693
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/85591
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
diff --git a/DEPS b/DEPS
index a3d7a67..0d1fc65 100644
--- a/DEPS
+++ b/DEPS
@@ -115,7 +115,7 @@
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling pdfium_tests
   # and whatever else without interference from each other.
-  'pdfium_tests_revision': '165422e0e969be8f2ccb11ff5a0934e4af664ef3',
+  'pdfium_tests_revision': '3c4d506fe010630ddd74d9a5e288a406dd6886e5',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling skia
   # and whatever else without interference from each other.
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 3883c2a..955839c 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -299,7 +299,10 @@
 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(_(skia|skiapaths))?(_(win|mac|linux))?.pdf.#.png"""
+  NAME_expected(_(skia|skiapaths))?(_(win|mac|linux))?.pdf.#.png
+  The expected format used by _CheckPngNames() in testing/corpus/PRESUBMIT.py
+  must be the same as this one.
+  """
   expected_pattern = input_api.re.compile(
       r'.+_expected(_(skia|skiapaths))?(_(win|mac|linux))?\.pdf\.\d+.png')
   results = []