This is a reland of bb881c88
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}

Change-Id: I6eeb68cee3bd2dff0eb6696bbe43a4d602ab960d
Reviewed-on: https://chromium-review.googlesource.com/566458Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46548}