Remove support for AllowBelowNormalFromBrowser experiment group.

Remove support for AllowBelowNormalFromBrowser experiment group.

The experiment is complete and no noticeable difference was seen between
BELOW_NORMAL and IDLE, but IDLE was selected to be the one that sticks.

Experiment results, analysis, and conclusion @
https://docs.google.com/document/d/1EgwEv0bRSzX9sNnD0wmSodKIQOHXT-umUKKneb6sTFI/edit

BUG=459600
Committed: https://crrev.com/3d47343676ca798119b8a280275b8207c7e6e842
Cr-Commit-Position: refs/heads/master@{#339680}

[gab, 2015-07-21 15:04:52]

The CQ bit was checked by gab@chromium.org to run a CQ dry run

[gab, 2015-07-21 15:05:11]

gab@chromium.org changed reviewers:
+ thakis@chromium.org

[gab, 2015-07-21 15:05:11]

Nico PTAL.

Thanks,
Gab

[commit-bot: I haz the power, 2015-07-21 15:05:26]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1249683002/1

[Nico, 2015-07-21 15:13:29]

lgtm

[commit-bot: I haz the power, 2015-07-21 16:20:11]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-21 16:20:12]

Dry run: This issue passed the CQ dry run.

[gab, 2015-07-21 16:56:40]

The CQ bit was checked by gab@chromium.org

[commit-bot: I haz the power, 2015-07-21 16:57:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1249683002/1

[commit-bot: I haz the power, 2015-07-21 17:20:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-21 17:22:25]

Patchset 1 (id:??) landed as
https://crrev.com/3d47343676ca798119b8a280275b8207c7e6e842
Cr-Commit-Position: refs/heads/master@{#339680}