Get rid of custom code for HTMLLinkElement.sizes attribute setter

Get rid of custom code for HTMLLinkElement.sizes attribute setter by
leveraging bindings generator support for [PutForwards=x] IDL extended
attribute support. The IDL now matches the specification as well:
http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#htmllinkelement

BUG=307045
R=haraken
TEST=fast/dom/icon-size-property.html

Review URL: https://codereview.chromium.org/27509002

git-svn-id: svn://svn.chromium.org/blink/trunk@159816 bbb929c8-8fbe-4397-9dbb-9b2b20218538