Roll PDFium 20fc82e..b8a0747

https://pdfium.googlesource.com/pdfium.git/+log/20fc82e..b8a0747

BUG=507316,537780,538103
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1381113003

Cr-Commit-Position: refs/heads/master@{#352125}