It is possible for JS objects to be allocated while we are retrieving the
profile. These JS objects can in turn end up getting sampled by the profiler.
Adding these to the profile data structures invalidates the iterators that
are presently in flight. This change prevents such concurrent modifications
from affecting the retrieve operation.

BUG=

Review URL: https://codereview.chromium.org/1735733002

Cr-Commit-Position: refs/heads/master@{#34298}