This is a reland of 69a502ce

TBR=ulan@chromium.org

Original change's description:
> [heap profiler] Refactor: Replace indices with HeapEntry*
>
> Change-Id: I0c176f66711d45e2f59d527f3133a1afbf825ec3
> Reviewed-on: https://chromium-review.googlesource.com/1229613
> Commit-Queue: Alexei Filippov <alph@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56245}

Change-Id: I416da19eb38a1a55f5e2f5897062bc1ca454ac34
Reviewed-on: https://chromium-review.googlesource.com/1246802
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56250}