Prepare DefinePropertyAccessor for callback transitions.

Prepare DefinePropertyAccessor for callback transitions.

Although things are currently OK here, in the future it won't be enough to check
for the existence of a CALLBACKS result, we must additionally check that it
actually contains an accessor. In a nutshell: 'sed s/IsFound/IsProperty/' once
again...

Additionally, the control flow in DefinePropertyAccessor has been simplified by
using a helper function.

Committed: https://code.google.com/p/v8/source/detail?r=11305

[Michael Starzinger, 2012-04-13 09:26:11]

LGTM.