Roll PDFium from 8d477f73bf8b to a7858e6eb347 (8 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/8d477f73bf8b..a7858e6eb347

2020-06-10 tsepez@chromium.org Pass v8::Platform across public APIs.
2020-06-09 thestig@chromium.org Make length parameter in FPDFAttachment_SetFile() non-const.
2020-06-09 nigi@chromium.org Add expected results for Skia in 2 embedder tests.
2020-06-09 nigi@chromium.org Consolidate repetitive constants in fpdf_edit_embeddertest.cpp.
2020-06-08 tsepez@chromium.org Pump V8 foreground tasks queued to platform message loop.
2020-06-08 awscreen@chromium.org Roll third_party/zlib/ 21c6af6f1..a68151fd9 (11 commits)
2020-06-08 thestig@chromium.org Simplify more code in CFDE_TextOut.
2020-06-08 awscreen@chromium.org Roll v8/ 6a91312c4..9912e35d5 (629 commits)

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: None
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I0dbf90a8f944b883564935cb2d962dcb4e878e91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239095Reviewed-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@{#776803}

DEPS

@@ -202,7 +202,7 @@ 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': '8d477f73bf8bc4f618e83b46a73cd5f7765e43b8',
+  'pdfium_revision': 'a7858e6eb347fced5d5f131e45c0b885f62a89e4',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.