76217f57 fixed the profiler so it would only sample a thread if
it had the Isolate lock. Unfortunately, this fix missed a timing
window where a thread might have the Isolate lock but might not
have restored the thread-specific data such as thread_local_top_
for the locked thread yet, so the sampler might end up using data
from a different thread.

This doesn't cause any seg faults or the like because the thread
we *meant* to sample has the Isolate lock so the thread we're
accidentally sampling can't mess with any Isolate data but we can
still get incorrect sample data which can be especially obvious if
the accidentally sampled thread is inside code that would never
run on the thread we meant to sample.

Fortunately, we can tell when all thread-specific data has been
restored to the Isolate because thread_state_ in the
PerIsolateThreadData for a thread is set to a non-null value
until everything has been restored, at which point it gets set
to null. So the fix adds a check after the test for the Isolate
lock to check if thread_state_ is null for the thread we mean to
sample. If so, we know all the data in the Isolate is good to go
for sampling.

Bug: v8:11316
Change-Id: I02d6361d8cbd6ec809ad8fb7ef07f5e9c94c7d1e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2628133Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72112}