Take more current safe_math_impl.h from upstream.
Take more current safe_math_impl.h from upstream.

Corresponds to version dfd77a987650965071d0fddfbe0b806ce62ba337.
Major change is to handle div by 0 without exceptions.
Safe shift is not yet present.

TBR=thestig@chromium.org
TBR=jschuh@chromium.org

Review-Url: https://codereview.chromium.org/2473513002

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