Remove TreatNullAs=NullString for HTMLAnchorElement

http://whatwg.org/html#htmlanchorelement
http://whatwg.org/html#HTMLAnchorElement-partial

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

Firefox reflects ping as a URL property, which is not per spec. 

IE11 reflects coords as '0,0,0,0' and shape as 'rect', while it doesn't
support ping/download.

Presto doesn't support ping/download.

BUG=310298

Review URL: https://codereview.chromium.org/58493002

git-svn-id: svn://svn.chromium.org/blink/trunk@161336 bbb929c8-8fbe-4397-9dbb-9b2b20218538