Revert 200932 "Lower the priority of shared workers that aren't ..."

Revert 200932 "Lower the priority of shared workers that aren't ..."

> Lower the priority of shared workers that aren't associated with the foreground tab.
> 
> This should help out less powerful devices in the case where there's a shared worker in another tab and a cpu intensive page in the foreground.
> 
> BUG=
> TEST=Open a doc, see webworker running, switch tabs and check webworker's priority by outputting contents of /sys/fs/cgroup/cpu/chrome_renderers/background/cgroup.proc on ChromeOS. With an open doc in a background tab, run 720p video (ie. youtube) at fullscreen, should stay fairly smooth.
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=199840
> 
> Review URL: https://chromiumcodereview.appspot.com/14137016

TBR=simonhatch@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=201090

[shatch, 2013-05-20 16:00:44]

Committed patchset #1 manually as r201090.