Fix a crash/hang that occurred when deleting a snapshot during the
GC that is part of taking another one.

Specifically, when deleting the only other snapshot in such
a situation, the `v8::HeapSnapshot::Delete()` method sees that there
is only one (complete) snapshot at that point, and decides that it is
okay to perform “delete all snapshots” instead of just deleting
the requested one. That resets the internal string lookup table
of the heap profiler, but the new snapshot that is currently in
progress still holds references to the old string lookup table,
leading to a use-after-free segfault or infinite loop.

Fix this by guarding against resetting the string table while
another heap snapshot is being taken, and add a test that would
crash before this fix.

This can be triggered in Node.js by repeatedly calling
`v8.getHeapSnapshot()`, which provides heap snapshots as weakly
held host objects.

Change-Id: If9ac3728bf79114000982f1e7bb05e8034299e3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2464823Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70445}