The index is an {int} initially. We then store it as {intptr_t}, and
the accessor returns it as {size_t}.
This CL consolidates everything to {int}, fixes naming of
{HasTrapHandlerIndex} and defines the simple accessors inline.

R=titzer@chromium.org

Bug: v8:9183
Change-Id: I1afa792117201d4dda3fcc437a4e518489b9ff17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1590079Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61146}