This is a minor refactoring before fixing actual issues.

1) The update of the {per_isolate_data_} is moved into
   {FloodWithBreakpoints}, which is already taking the mutex.
2) The {PrepareStep} method takes a {WasmFrame*} directly instead of its
   ID. In most cases, this prevents the creation of an additional stack
   frame iterator.

R=thibaudm@chromium.org

Bug: chromium:1145176
Change-Id: I1a6cd15550bbb4ef78ba522427bed1c23185569e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2558318Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71399}