Add SendSignalWithNoArgumentsToSystemBus() to dbus.h

The original code is from libcros's chromeos_screen_lock.cc.
We should have this in a common place so we can use this from elsewhere.

BUG=chromium-os:5763
BUG=chromium-os:6526

Change-Id: Ia703a18b2344013a01bc136d9c8d320a45c9dedc

TEST=Confirmed that the screen locker works as before with this fucntion.

Review URL: http://codereview.chromium.org/3922003
2 files changed