This reverts commit d0e3fb4f.

Reason for revert: Failures on all GPU bots:
https://build.chromium.org/p/client.v8.fyi/builders/Win%20Release%20%28NVIDIA%29/builds/3171
https://build.chromium.org/p/client.v8.fyi/builders/Linux%20Release%20%28NVIDIA%29/builds/3730

Original change's description:
> Reland: [heap] Add missing steps for Add/Remove observers
> 
> This is a reland of https://chromium-review.googlesource.com/c/v8/v8/+/715118.
> The UI doesn't allow me to reland this automatically due to merge conflicts
> so I ended up doing this manually.
> 
> Change-Id: Ic013b61f459cb1e91e24203fb31ae7c14f3754f8
> Reviewed-on: https://chromium-review.googlesource.com/720136
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
> Cr-Commit-Position: refs/heads/master@{#48606}

TBR=ulan@chromium.org,mlippautz@chromium.org,ofrobots@google.com

Change-Id: I80f63ac0cc54471b3ecaa7b6d85b1805ea599dd4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/721440Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48614}