Instead of keeping a single {stepping_frame_} per native module, we now
keep one frame id per isolate. Hence, each isolate can step through a
different frame, independent of other isolates.
The on-stack-replacement of the stepping frame already works on a
per-isolate basis, since we only replace the return address of a single
frame, part of the isolate that requested stepping.

The new test (which also executes in a variant with two concurrent
isolates) revealed some more data races to fix.

R=thibaudm@chromium.org

Bug: v8:10359
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: I0bb013737162bd09b9f4be9c08990bca7bf736ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2214838Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68045}