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Windows has a fancy backgrounding mode which lowers IO priority.
Sadly it can only be set from within the process to be backgrounded.
So add a new ChildProcessMsg to tell child processes when they should
enter and leave background mode.

This reduces glitching during background tab load. This should only be
landed along with https://codereview.chromium.org/298253004/ which
removes backgrounding for active audio tabs.

I expect this will reduce CPU and I/O load significantly for background
processes which will lead to power savings.

This is only done on Windows since ChromeOS / Linux require privileges
only available to the browser process for setting background mode.

BUG=362294
TEST=Play audio. Load background tabs. Observe less/no glitching.
R=jam@chromium.org, luken@chromium.org, nasko@chromium.org

Review URL: https://codereview.chromium.org/305533006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274071 0039d316-1c4b-4281-b951-d872f2087c98