Roll PDFium 7c9e452..640c395

      https://pdfium.googlesource.com/pdfium.git/+log/7c9e452..640c395

BUG=522131
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1307343004

Cr-Commit-Position: refs/heads/master@{#347271}