Roll src/third_party/pdfium 7b8eb884cfd6..a451858403a2 (3 commits)

https://pdfium.googlesource.com/pdfium.git/+log/7b8eb884cfd6..a451858403a2

git log 7b8eb884cfd6..a451858403a2 --date=short --first-parent --format='%ad %ae %s'
2020-01-31 thestig@chromium.org Remove some CPDF_ImageLoader::Start() parameters.
2020-01-31 tsepez@chromium.org Remove CFFL_FormFiller::WndtoPWL() and FFLtoWnd()
2020-01-31 dhoss@chromium.org Add test case for dysfunctional FPDFTextObj_SetTextRenderMode()

Created with:
  gclient setdep -r src/third_party/pdfium@a451858403a2

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: None
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: Ifae40f85d36abadc7d886f7125a0ffb2132efc5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033602Reviewed-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@{#737547}

DEPS

@@ -197,7 +197,7 @@ 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': '7b8eb884cfd6da446798014671be54bc2fed305e',
+  'pdfium_revision': 'a451858403a2ff5e9302980cbf813455ec05ef7b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.