Roll PDFium 2fad11a..df6ec80

https://pdfium.googlesource.com/pdfium.git/+log/2fad11a..df6ec80

BUG=612918,619405,621094
TBR=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2078383003
Cr-Commit-Position: refs/heads/master@{#400811}