We will use this state in devtools via the inspector to indicate
whether a thread is currently stuck polling in atomics.wait.

VMState already distinguishes the important states we care about which
are idle vs. running JS. We also want to know the state for
atomics.wait(), which is commonly used in WebWorkers to poll the main
page for work to do.

This CL just adds and maintains the state and adds assertions in
atomics tests. Another CL will emit inspector notifications when the
VMState changes in a way that the inspector cares about.

Re-flow comments as a drive-by cleanup.

Bug: chromium:1025490
Change-Id: I961051bfb846aa20454a56214310370ea8e47d1c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2033168
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66071}