Roll PDFium from 448f3994dc49 to 4a5e28a78c2d (2 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/448f3994dc49..4a5e28a78c2d

2023-05-19 kmoon@chromium.org Fix text string handling on UTF-32 platforms
2023-05-18 kmoon@chromium.org Improve ParseDecodeTest cases for test strings

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/pdfium-autoroll
Please CC dhoss@chromium.org,pdfium-deps-rolls@chromium.org,thestig@chromium.org on the revert to ensure that a human
is aware of the problem.

To file a bug in PDFium: https://bugs.chromium.org/p/pdfium/issues/entry
To file a bug in Chromium: https://bugs.chromium.org/p/chromium/issues/entry

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Bug: None
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I41931826ae3d823912a9ca6545bb0d8fab3d725a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549509
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Bot-Commit: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1146748}