Revert of Debugger: use a Map to cache mirrors.

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

Committed: https://crrev.com/1f2c5052ce859d20ea75af1da270e18d6ecacc39
Cr-Commit-Position: refs/heads/master@{#30153}

[Yang, 2015-08-13 12:09:17]

Created Revert of Debugger: use a Map to cache mirrors.

[commit-bot: I haz the power, 2015-08-13 12:09:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1292023002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1292023002/1

[commit-bot: I haz the power, 2015-08-13 12:09:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-13 12:09:50]

Patchset 1 (id:??) landed as
https://crrev.com/1f2c5052ce859d20ea75af1da270e18d6ecacc39
Cr-Commit-Position: refs/heads/master@{#30153}