This reverts commit bb881c88.

Reason for revert: nosnap debug:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap%20-%20debug/builds/13930

Original change's description:
> Document RootIndexMap's requirement for immovable roots.
> 
> R=​mstarzinger@chromium.org
> 
> Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
> Reviewed-on: https://chromium-review.googlesource.com/565281
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46517}

TBR=yangguo@chromium.org,mstarzinger@chromium.org

Change-Id: Ifac3ec6192207bf2be8932408c37bea66bb435e8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/565598Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46522}