We stored the size of a function body before we check that
these values are valid. This caused a failing DCHECK in the constructor
of WireBytesRef which checked for integer overflows. With this CL we
check the size of the function body before we create the WireBytesRef.

R=clemensh@chromium.org

Bug: chromium:738097
Change-Id: I18f8b628c1499aae9c8e9340ea73c87f19e6f1d7
Reviewed-on: https://chromium-review.googlesource.com/561000
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46442}