Update config to add a new state to custom tab experiment

Update config to add a new state to custom tab experiment

This add another state to disable prerendering in custom tabs
while keeping the rest of the feature enabled adding another
level of control to the kill switch.

BUG=493656
Committed: https://crrev.com/60fe84a70f7e6e65ff5a3cb3f96c1b39445e47f9
Cr-Commit-Position: refs/heads/master@{#336184}

[Yusuf, 2015-06-25 00:14:04]

yusufo@chromium.org changed reviewers:
+ dfalcantara@chromium.org

[gone, 2015-06-25 00:53:25]

lgtm

[Yusuf, 2015-06-25 16:59:36]

The CQ bit was checked by yusufo@chromium.org

[commit-bot: I haz the power, 2015-06-25 17:00:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1203203004/1

[commit-bot: I haz the power, 2015-06-25 17:12:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-25 17:13:57]

Patchset 1 (id:??) landed as
https://crrev.com/60fe84a70f7e6e65ff5a3cb3f96c1b39445e47f9
Cr-Commit-Position: refs/heads/master@{#336184}