This CL includes validation code for the trap handler data structures in debug
mode to help catch issues like v8:6841 sooner in the future.

We also now eagerly initialize the free list pointers to make the logic of
finding the next free entry more obvious.

Bug: v8:5277
Change-Id: I13c3180c59b6152508c480e2042072a91e6ca977
Reviewed-on: https://chromium-review.googlesource.com/674128
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48186}