Revert of Debugger: use a Map to cache mirrors. (patchset #1 id:1 of https://codereview.chromium.org/1287243002/ )

Reason for revert:
Several nosnap and custom snapshot failures.

Original issue's description:
> Debugger: use a Map to cache mirrors.
>
> This makes mirror cache lookup O(1) instead of O(n).
> The downside is that the lookup via handle is O(n). This
> is fine because handles are only used in the JSON api,
> which is not used by Chrome and on death row.
>
> Committed: https://crrev.com/890b1dfca84d9dfecdcfc56517ef541076c6eb1d
> Cr-Commit-Position: refs/heads/master@{#30150}

TBR=bmeurer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1292023002

Cr-Commit-Position: refs/heads/master@{#30153}