Add test coverage for --prerender=disabled switch

Add test coverage for --prerender=disabled switch

This adds a test to ensure that the disable switch is respected
by ConfigurePrerender in prerender_field_trial.cc.

BUG=455449
Committed: https://crrev.com/0fd78680ee76c85d5cf9eec9c15a30a99d0520a3
Cr-Commit-Position: refs/heads/master@{#315336}

[dspell, 2015-02-05 00:25:48]

dspell@amazon.com changed reviewers:
+ cbentzel@chromium.org

[dspell, 2015-02-05 00:25:49]

Hello cbentzel, this is a proposed test for BUG=455449. Can you take a look when
you have the chance? Feel free to suggest other reviewers.

Thanks,
dspell

[cbentzel, 2015-02-09 13:25:35]

LGTM

Apologies for delay

[dspell, 2015-02-09 17:15:12]

The CQ bit was checked by dspell@amazon.com

[commit-bot: I haz the power, 2015-02-09 17:15:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/897983003/1

[commit-bot: I haz the power, 2015-02-09 17:56:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-09 17:57:14]

Patchset 1 (id:??) landed as
https://crrev.com/0fd78680ee76c85d5cf9eec9c15a30a99d0520a3
Cr-Commit-Position: refs/heads/master@{#315336}