Replace ad-hoc weakness in prototype transitions with WeakCell.

Replace ad-hoc weakness in prototype transitions with WeakCell.

BUG=

Committed: https://crrev.com/bfb81fbe0d38c88cc1b6ed6eb1a4b8cbe2c469c3
Cr-Commit-Position: refs/heads/master@{#28830}

[ulan, 2015-06-03 14:11:18]

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

[ulan, 2015-06-03 14:11:18]

ptal

[Hannes Payer (out of office), 2015-06-03 18:04:41]

cool! lgtm

[Jakob Kummerow, 2015-06-08 08:51:57]

lgtm

[ulan, 2015-06-08 10:00:58]

The CQ bit was checked by ulan@chromium.org

[ulan, 2015-06-08 10:00:58]

The patchset sent to the CQ was uploaded after l-g-t-m from hpayer@chromium.org
Link to the patchset: https://codereview.chromium.org/1163073002/#ps20001
(title: "Fix crash")

[commit-bot: I haz the power, 2015-06-08 10:01:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1163073002/20001

[commit-bot: I haz the power, 2015-06-08 10:20:39]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-08 10:21:00]

Patchset 2 (id:??) landed as
https://crrev.com/bfb81fbe0d38c88cc1b6ed6eb1a4b8cbe2c469c3
Cr-Commit-Position: refs/heads/master@{#28830}

[ulan, 2015-06-08 11:20:04]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/1169733002/ by ulan@chromium.org.

The reason for reverting is: GC stress failures..