Debugger: use a Map to cache mirrors.

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}

[Yang, 2015-08-13 08:20:41]

yangguo@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2015-08-13 08:51:59]

LGTM.

[Yang, 2015-08-13 09:54:19]

FYI yurys@

[Yang, 2015-08-13 10:21:37]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-08-13 10:21:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287243002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287243002/1

[commit-bot: I haz the power, 2015-08-13 10:49:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-13 10:49:25]

Patchset 1 (id:??) landed as
https://crrev.com/890b1dfca84d9dfecdcfc56517ef541076c6eb1d
Cr-Commit-Position: refs/heads/master@{#30150}

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1292023002/ by yangguo@chromium.org.

The reason for reverting is: Several nosnap and custom snapshot failures..