This implements a proof-of-concept sampling based heap profiler inspired by
tcmalloc's heap profiler [1] and Go's mprof/memprofile [2].

The basic idea is the sample allocations using a randomized Poisson process. At
any point in time we can cheaply request the set of live sample objects that
should be a representative sample of heap. Samples include stack-traces from the
allocation sites, making this an effective tool for memory leak debugging.

Unlike AllocationTracking, this is intended to be cheap and usable online in
production.

The proof-of-concept is only sampling new-space allocations at this point.
Support for sampling paged space and native allocations is anticipated in the
future.

[1] http://goog-perftools.sourceforge.net/doc/heap_profiler.html
[2] http://blog.golang.org/profiling-go-programs

Review URL: https://codereview.chromium.org/1555553002

Cr-Commit-Position: refs/heads/master@{#33448}