Enable WebUI ScreenLocker for aura by abstracting lock window.
This CL abstracts the actual lock window allowing for a GTK implementation and an aura implementation. The aura screenlocker stubs are removed and the screenlocker is enabled on aura using the aura lock window.
BUG=
100723
TEST=Build chrome with use_aura=1, trigger screen lock and test unlocking screen.
Committed:
http://src.chromium.org/viewvc/chrome?view=rev&revision=112709