Roll src/third_party/pdfium 6af2de8b2aa5..4c0fac2e6b05 (8 commits)

https://pdfium.googlesource.com/pdfium.git/+log/6af2de8b2aa5..4c0fac2e6b05

git log 6af2de8b2aa5..4c0fac2e6b05 --date=short --first-parent --format='%ad %ae %s'
2020-04-17 thestig@chromium.org Replace CPDF_CharPosList class with a standalone function.
2020-04-17 thestig@chromium.org Mark CFX_Font::IsSubstFontBold() as Skia-only.
2020-04-17 thestig@chromium.org Add a basic embedder test for FPDF_LoadMemDocument64().
2020-04-17 thestig@chromium.org Reorder parameters in PWL and related events code.
2020-04-17 thestig@chromium.org Merge and extend FocusableAnnotSubtypes tests.
2020-04-17 thestig@chromium.org Assert timer IDs are unique.
2020-04-17 thestig@chromium.org Call FORM_DoDocumentAAction() in EmbedderTest::CloseDocument().
2020-04-17 tsepez@chromium.org Put remaining uint16_t vectors into data partition

Created with:
  gclient setdep -r src/third_party/pdfium@4c0fac2e6b05

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/+/master/autoroll/README.md

Bug: chromium:1071689
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I984d04b182f19dd40674e930499371c901d22c74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154977Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#760255}

DEPS

@@ -195,7 +195,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.
-  'pdfium_revision': '6af2de8b2aa5819d76ec8f3d533b09c7b39fab49',
+  'pdfium_revision': '4c0fac2e6b05f8a4008584fd38f22ab1eae364f9',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.