Try to make a NOTREACHED definition suitable for win8.

See http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_gn_rel/builds/5674

Testing this is a two-step process: land CL to pdfium, then try DEPS roll
in chromium against this version on the win8 trybots.

TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1004733006