Bug 836050 part 1.  Make Element::GetAttr non-virtual.  r=smaug
Bug 836050 part 2.  Make Element::HasAttr non-virtual.  r=smaug
Bug 836050 part 3.  Make Element::AttrValueIs non-virtual.  r=smaug
Bug 836050 followup.  Rev the nsIContent and Element IIDs.  r=smaug