Only have a single call into pdfium at a time.

Pdfium is not thread safe and uses global variables, hence no
parallel call pdfium is safe

Fixes: 28705066
Change-Id: I04309ee691bd9cea37587e0af5be2c07ce8c9f2c
(cherry picked from commit 0768a7dc450caf4c873c5b0883a75135536e1546)
4 files changed