If multiple isolates are using the same module, we need to keep it
tiered down as long as any isolate still has a debugger open.
Also, we cannot short-cut the {NativeModule::TierDown} method, since the
previously triggered tier down might not have finished yet.
For now, each isolate starts an independent tier down (i.e. a full
recompilation). We could optimize this later by skipping functions that
are already tiered down, or are already scheduled for tier down, but we
still need to wait for tier-down to finish on each isolate.

R=thibaudm@chromium.org

Bug: v8:10359
Change-Id: I7ea6a6f5d3977e48718ac5bc94f9831541f6173f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190758
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67716}