A small refactoring to reduce complexity. It also moves the call to
{RegisterTrapHandlerData} out of the mutex to reduce the time in this
critical section.

R=mstarzinger@chromium.org

Bug: v8:8916
Change-Id: I644f03db6099ebef22b2e33b607a2dc038b36423
Reviewed-on: https://chromium-review.googlesource.com/c/1478196
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59907}