Switch to ValueOrDie
Switch to ValueOrDie

We know the values are always valid at this point, so use ValueOrDie instead
of ValueOrDefault.

BUG=chromium:637984

Review-Url: https://codereview.chromium.org/2319343003

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium