This is a reland of c34042cc
Original change's description:
> [heap-profiler] Do not treat WeakMap values as weak.
> 
> For the WeakHashTable objects only mark keys as weak while leaving values as strong references.
> 
> BUG=chomium:773722
> 
> Change-Id: Iabd5ba293d05fe68a2af6503fcdd711ecc182482
> Reviewed-on: https://chromium-review.googlesource.com/730771
> Commit-Queue: Alexei Filippov <alph@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48985}

TBR=ulan@chromium.org

Bug: chomium:773722
Change-Id: Icede16fde528d147cde5c3f6c72f2029876b099f
Reviewed-on: https://chromium-review.googlesource.com/740722Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48987}