Add Element::subResourceAttributeName() virtual function

Add Element::subResourceAttributeName() virtual function to distribute the code
to each HTML class, instead of having a complex function with a lot of branching
to determine the subresource attribute name of an Element.

R=tkent, adamk

Review URL: https://codereview.chromium.org/202633009

git-svn-id: svn://svn.chromium.org/blink/trunk@169558 bbb929c8-8fbe-4397-9dbb-9b2b20218538