Update the Skia expectation for mask_array.pdf

CL [1] fixed the transparency level of the image in mask_array.pdf.
This CL updates the Skia expectation for this test.

[1] https://pdfium-review.googlesource.com/96812

Bug: pdfium:1861
Change-Id: I63e10b52fe550d4525321df18dd92e1dfc6710c0
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/97990
Reviewed-by: Lei Zhang <thestig@chromium.org>