Roll PDFium from 4adcb08d8133 to 20b8b48e460b (4 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/4adcb08d8133..20b8b48e460b

2025-01-09 tsepez@chromium.org Avoid UNSAFE_TODO() in AreColorIndicesOutOfBounds().
2025-01-09 tsepez@chromium.org Remove distinction between input/output views in fxcrt::Zip().
2025-01-09 tsepez@chromium.org Update documentation and tests for fxcrt::Zip()
2025-01-09 tsepez@chromium.org Make PDFium's compiler_specific.h use clang's UNSAFE_BUFFERS_BUILD

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 andyphan@google.com,dhoss@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://issues.skia.org/issues/new?component=1389291&template=1850622

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

Bug: None
Tbr: andyphan@google.com
Change-Id: Id69449928941a4fe60d56862919b5482a62fc0e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6165934
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@{#1404533}