When a tab not playing audio is sent to the background, set timer slack to its maximum value.

Support for setting timer slack is added at the MessageLoop level, the concrete implementation of this CL only affects CFMessagePump backed MessageLoops (which means just the main thread for backgrounded renderer processes at present).

The MessageLoop implementation is designed to support its use on Windows and Linux (the Windows API sets slack per-timer like the Mac one, while on Linux slack, is set per-thread via a call to prctl() using PR_SET_TIMERSLACK).

BUG=356804

Review URL: https://codereview.chromium.org/289863005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275218 0039d316-1c4b-4281-b951-d872f2087c98