It isn't expensive to set this enum on the isolate and we only call
this in chrome when the ThreadDebugger is enabled anyway.

This means we have a correct idle state to use with the upcoming
CDP event which emits the thread state.

Bug: chromium:1025490
Change-Id: Ia555ac8c401d0ad7c72d4cbc442be388ce166131
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2036087Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66125}