Roll src/third_party/pdfium 54eda5713da0..6524ae660745 (2 commits)

https://pdfium.googlesource.com/pdfium.git/+log/54eda5713da0..6524ae660745

git log 54eda5713da0..6524ae660745 --date=short --no-merges --format='%ad %ae %s'
2019-10-30 tsepez@chromium.org Prevent duplicate string creation in CFWL_ComboBox::AddString()
2019-10-30 tsepez@chromium.org Remove c_str() calls from CXFA_Node::InsertItem()

Created with:
  gclient setdep -r src/third_party/pdfium@6524ae660745

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

TBR=pdfium-deps-rolls@chromium.org

Bug: None
Change-Id: Idc4e822f8c6b0a99586859d2b84dcbc786dd7275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892391Reviewed-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@{#711135}

DEPS

@@ -192,7 +192,7 @@ 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': '54eda5713da0908738c79021da5314b91fb773ae',
+  'pdfium_revision': '6524ae6607454e1a40fdc85ab79b85161296bb40',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.