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This makes the system tray the first child so that it is the first tray focused when the status area widget is selected via accessibility.

This depends on http://codereview.chromium.org/10823350/

BUG=142506
For ash/ash_strings.grd:
TBR=sky@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10825389

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152135 0039d316-1c4b-4281-b951-d872f2087c98