Revert "Revert 239730 "ash_system_tray_delegate -> system_tray_delegate_...""

This reverts 240379..

BUG=311373
TBR=stevenjb

Review URL: https://codereview.chromium.org/126213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243420 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243420