Roll PDFium from b25f2f06432b to 8c187c36bed6 (8 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/b25f2f06432b..8c187c36bed6

2021-05-12 thestig@chromium.org Fix name style for CPDF_DataAvail enums.
2021-05-12 thestig@chromium.org Clean up CPDF_DataAvail.
2021-05-12 thestig@chromium.org Check for repeated pages in CPDF_DataAvail::GetPageKids().
2021-05-12 tsepez@chromium.org Make FPDF_AVAIL type-safe across public API.
2021-05-12 tsepez@chromium.org Make embeddertests check fpdf_annot.h documented flag return values.
2021-05-12 tsepez@chromium.org Avoid global cppgc:Persistents<> in tests with multiple heaps.
2021-05-12 tsepez@chromium.org Remove unused member from GCedTreeNodeMixinUnitTest.
2021-05-12 tsepez@chromium.org Pass all flags to FPDF_SaveWithVersion().

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 pdfium-deps-rolls@chromium.org on the revert to ensure that a human
is aware of the problem.

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/master/autoroll/README.md

Bug: chromium:1201087
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I18f7d582f9f4580ee8e9901f38c7933749928c43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2893594
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/master@{#882426}