Roll src/third_party/pdfium 1c5647025500..951c1df3ce7d (5 commits)

https://pdfium.googlesource.com/pdfium.git/+log/1c5647025500..951c1df3ce7d

git log 1c5647025500..951c1df3ce7d --date=short --first-parent --format='%ad %ae %s'
2020-02-13 tsepez@chromium.org Use Optional<> return in CheckStyleFromCaption().
2020-02-13 thestig@chromium.org Tidy CPDF_FormControl.
2020-02-13 thestig@chromium.org Fix regression with CheckStyleFromCaption().
2020-02-13 hikakar@microsoft.com Fix the rendering of scientific symbols correctly
2020-02-13 tsepez@chromium.org Tidy CFX_CSSSyntaxParser, part 2.

Created with:
  gclient setdep -r src/third_party/pdfium@951c1df3ce7d

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: I0fe7da575c1bb11c1fa5ffa9b431d41715a9ddb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055054Reviewed-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@{#741189}

DEPS

@@ -191,7 +191,7 @@ 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': '1c5647025500b429c1dc5e5175b019d07ea38407',
+  'pdfium_revision': '951c1df3ce7d5c44e5533b900eb23428b3b0f92c',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.