Revert of Disable BackgroundRendererProcesses in absence of Finch.

Revert of Disable BackgroundRendererProcesses in absence of Finch. (patchset #1 id:1 of https://codereview.chromium.org/1076033002/)

Reason for revert:
Bring BackgroundRendererProcesses back on trunk -- will give us perf waterfall coverage now (session_restore tests were borked when this landed so we didn't get the backward effect and are still in the dark here)...

Original issue's description:
> Disable BackgroundRendererProcesses experiment in absence of Finch.
>
> (temporarily in order to get perf waterfall coverage for new session restore tests)
>
> BUG=459600
>
> Committed: https://crrev.com/78011fcdf7c4e17949429bb99fbe6a68ca822b44
> Cr-Commit-Position: refs/heads/master@{#324521}

TBR=avi@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=459600
Committed: https://crrev.com/5e19adbbd40cc34e4b78ab7b337f39a13708b6ad
Cr-Commit-Position: refs/heads/master@{#324838}

[gab, 2015-04-13 12:45:16]

Created Revert of Disable BackgroundRendererProcesses in absence of Finch.

[commit-bot: I haz the power, 2015-04-13 12:45:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1083673002/1

[commit-bot: I haz the power, 2015-04-13 12:46:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-13 12:46:59]

Patchset 1 (id:??) landed as
https://crrev.com/5e19adbbd40cc34e4b78ab7b337f39a13708b6ad
Cr-Commit-Position: refs/heads/master@{#324838}