Revert of Don't choose a background process when choosing a process for starting a Service Worker.

Revert of Don't choose a background process when choosing a process for starting a Service Worker. (patchset #2 id:20001 of https://codereview.chromium.org/1373853003/ )

Reason for revert:
- This patch will increase memory pressure to launch a new SW process.
- More reasonable patch (crrev.com/351714) landed.

Original issue's description:
> Don't choose a background process when choosing a process for starting a Service Worker.
>
> This is a quick fix for https://crbug.com/531345.
> We need to revisit the behavior of ServiceWorker and the timer suspension of background processes which was intoruced by https://codereview.chromium.org/1133143003.
>
> BUG=531345
>
> Committed: https://crrev.com/3ddd919b0f5550377e178ba6d4aa5256701d53ff
> Cr-Commit-Position: refs/heads/master@{#351530}

TBR=kinuko@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=531345
Committed: https://crrev.com/fc61da261fddeb5e9cc65bc93b3620258a596dce
Cr-Commit-Position: refs/heads/master@{#351720}

[horo, 2015-10-01 03:43:43]

Created Revert of Don't choose a background process when choosing a process for
starting a Service Worker.

[commit-bot: I haz the power, 2015-10-01 03:43:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1380023002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1380023002/1

[commit-bot: I haz the power, 2015-10-01 03:44:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-01 03:45:46]

Patchset 1 (id:??) landed as
https://crrev.com/fc61da261fddeb5e9cc65bc93b3620258a596dce
Cr-Commit-Position: refs/heads/master@{#351720}