Remove some useless null checks in Element/NamedNodeMap
    
Make shouldIgnoreAttributeCase() a method of Element, thus avoiding
unnecessary element null checks.
    
This is based on WebKit r122767 by <akling@apple.com>.
    
R=tkent

Review URL: https://codereview.chromium.org/26955005

git-svn-id: svn://svn.chromium.org/blink/trunk@159789 bbb929c8-8fbe-4397-9dbb-9b2b20218538