Fix GN build under chromium checkout.

Original patch at https://codereview.chromium.org/817813004
This suppresses the fatal compilation warnings introduced by
https://pdfium.googlesource.com/pdfium/+/e4fc5ced45c8fcfbe2487ec64eab036bc7d57602

TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/804253005
1 file changed