Bug 781360 patch 1:  Rename nsStyleContext::GetRuleNode to RuleNode, since it can never return null.  r=dholbert

This makes it conform to our convention that getters returning pointers
that can never be null do not begin with "Get".

nsStyleContext's rule node is never null because we require a rule node
in order to construct a style context.
Bug 781360 patch 2:  Rename nsRuleNode::GetPresContext to PresContext, since it can never return null.  r=dholbert