Make HTMLAnchorElement.text getter behave according to specification

Make HTMLAnchorElement.text getter behave according to specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#dom-a-text

According to the specification, the 'text' IDL attribute, on getting, must
return the same value as the textContent IDL attribute on the element. However,
Chromium was returning 'innerText' instead. Upon setting, Chromium was
correctly updating the 'textContent' attribute though.

The new behavior is consistent with Firefox 29 and IE11.

This CL is based on arv's comment at:
https://codereview.chromium.org/263353004/#msg2

R=arv@chromium.org, tkent@chromium.org
BUG=369950
TEST=fast/dom/HTMLAnchorElement/get-text.html

Review URL: https://codereview.chromium.org/272693002

git-svn-id: svn://svn.chromium.org/blink/trunk@173579 bbb929c8-8fbe-4397-9dbb-9b2b20218538