Update expected png for XFA dropdown arrow without gradient.

Related to https://pdfium-review.googlesource.com/c/pdfium/+/28090

Bug: chromium:592748
Change-Id: If18002bfa251c555f725dffa240dc8f678ad710c
Reviewed-on: https://pdfium-review.googlesource.com/28130
Reviewed-by: dsinclair <dsinclair@chromium.org>

xfa_specific/fx/test_case_0025/Test_Drop_downList_expected.pdf.0.png

diff --git a/xfa_specific/fx/test_case_0025/Test_Drop_downList_expected.pdf.0.png b/xfa_specific/fx/test_case_0025/Test_Drop_downList_expected.pdf.0.png
index 073e4b5..f1254ea 100644
--- a/xfa_specific/fx/test_case_0025/Test_Drop_downList_expected.pdf.0.png
+++ b/xfa_specific/fx/test_case_0025/Test_Drop_downList_expected.pdf.0.png
Binary files differ