Implements poisson unsampling. A poisson process is used to determine
which samples to collect based on a sample rate. Unsampling will
approximate the true number of allocations at each site taking into
account that smaller allocations are less likley to be sampled.

This work was originally being done in the agent that
consumes profiles but it is more efficient to do it here
and individual consumers of the API should not have to
worry about the mathematical details of the sampling
process.

R=ofrobots@google.com
BUG=

Review URL: https://codereview.chromium.org/1706343002

Cr-Commit-Position: refs/heads/master@{#34234}