Replace elementHasLegalLinkAttribute() helper with virtual function

Replace elementHasLegalLinkAttribute() helper with virtual function on Element:
hasLegalLinkAttribute(). elementHasLegalLinkAttribute() was a bit complex due
to all the branching. Using a virtual function distributes the code to each
class and results in clearer code.

R=tkent, adamk

Review URL: https://codereview.chromium.org/202893002

git-svn-id: svn://svn.chromium.org/blink/trunk@169412 bbb929c8-8fbe-4397-9dbb-9b2b20218538