If no objects allocated at a location are live when a profile is
collected we report a zero count sample. This is confusing to those
looking at the profiles and will leak memory.

We now delete allocations once the number of sampled live objects for
that location reaches zero.

R=ofrobots@google.com
BUG=

Review URL: https://codereview.chromium.org/1828333002

Cr-Commit-Position: refs/heads/master@{#35305}