Roll src/third_party/pdfium 1575b47cd3bd..5540fdbe7451 (7 commits)

https://pdfium.googlesource.com/pdfium.git/+log/1575b47cd3bd..5540fdbe7451

git log 1575b47cd3bd..5540fdbe7451 --date=short --first-parent --format='%ad %ae %s'
2020-04-03 bravi@microsoft.com Modify test to validate tabbing over links and highlights
2020-04-03 tsepez@chromium.org Store uint8_t vector data in data partition in fxcodec/
2020-04-03 tsepez@chromium.org Store uint8_t vector data in data partition in fxge/
2020-04-03 thestig@chromium.org Consolidate more work into CFX_WideTextBuf::ExpandWideBuf().
2020-04-03 thestig@chromium.org Roll v8/ 633234050..1668abddd (139 commits)
2020-04-03 nigi@chromium.org Refactor image comparing code into _RunImageDiffCommand().
2020-04-03 bravi@microsoft.com Tests added for Action handling of enter key press over buttons.

Created with:
  gclient setdep -r src/third_party/pdfium@5540fdbe7451

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:1028991,chromium:994500
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I80bf6dffc717be0bcab83155fa382e445525503b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2136200Reviewed-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@{#756473}

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': '1575b47cd3bd239ff6cdd5ce7ec80585417f6a94',
+  'pdfium_revision': '5540fdbe74510620b27fc3cc29a720e766584189',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.