This reverts commit e770879e.

Reason for revert: broke on this build: https://build.chromium.org/p/client.v8/builders/V8%20Mac%20-%20debug/builds/16703

Original change's description:
> [heap] Add missing steps for Add/Remove observers
> 
> Change-Id: I9935ff4debc623af674e606c006085258b685ced
> Reviewed-on: https://chromium-review.googlesource.com/715118
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
> Cr-Commit-Position: refs/heads/master@{#48513}

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

Change-Id: Idc53870cbcb692e79764dfe0984ff765ed2458f3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/717318Reviewed-by: Ali Ijaz Sheikh <ofrobots@google.com>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#48515}