Make heap profiler provide information about each sample currently
alive. That information can be used to build diagrams of memory
allocations over time.

BUG=chromium:889545

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ibcbe2f5302263d0b3976ee4cd3601eff11375cae
Reviewed-on: https://chromium-review.googlesource.com/c/1285130
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56793}