Reland "Replace ad-hoc weakness in prototype transitions with WeakCell."

Reland "Replace ad-hoc weakness in prototype transitions with WeakCell."

Added missing slots recording.

This reverts commit fd2e3344d3116cd9ca11a14a1f4d800effba25ba.

BUG=

Committed: https://crrev.com/659ea368f754fbf678e376d546b9a4391e6913e5
Cr-Commit-Position: refs/heads/master@{#28857}

[ulan, 2015-06-08 12:01:03]

ulan@chromium.org changed reviewers:
+ hpayer@chromium.org

[ulan, 2015-06-08 12:01:03]

PTAL. The slots recording was omitted before because maps don't move. Now they
replaced by weakcells, which can move.

[Hannes Payer (out of office), 2015-06-08 21:37:31]

lgtm

[ulan, 2015-06-09 08:22:12]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-06-09 08:22:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169743002/20001

[commit-bot: I haz the power, 2015-06-09 09:00:14]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-09 09:00:23]

Patchset 2 (id:??) landed as
https://crrev.com/659ea368f754fbf678e376d546b9a4391e6913e5
Cr-Commit-Position: refs/heads/master@{#28857}