Sets an atomic field on each sampler when it requests a sample, to be
checked when the SIGPROF handler is executed. A counter is not used
since signals may be coalesced.

Prior to this change, all samplers attached to an isolate received
samples when other samplers sent SIGPROF to the VM thread. This change
alters the behaviour of different CpuProfiler instances on the same
isolate to be in line with the Windows / Fuchsia behaviour.

Bug: v8:8835
Change-Id: I0caaa845b596efc9d8b1cd7716c067d9a6359c57
Reviewed-on: https://chromium-review.googlesource.com/c/1468941
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59545}