Change SpecialMobileUserlogin::executeWhenAvailable() back to execute()

When I updated the method names in b1dae4fc, I accidentally updated the
method name in SpecialMobileUserlogin even though it is not a child of
MobileSpecialPage. Oops! Thanks for catching this, jdlrobson.

Change-Id: If9125440fefc5c1d14149e03de082631728b68d6