Maybe (defined in include/v8.h) is an API object, not meant to be used
in internal code. Instead of failing, it will call a callback on the
isolate.
It also adds one word to the size of each WasmCode object.

This CL avoids its use WasmCode. Instead, we use a sentinel value as the
function index of anonymous functions and add proper DCHECKs.

R=mstarzinger@chromium.org

Bug: v8:8015
Change-Id: I4bb155e814d8d0cc9e40b33202b4431718ac79b1
Reviewed-on: https://chromium-review.googlesource.com/1242096
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56222}