Get rid of special casing for SVGAElement.target from the bindings generator

Get rid of special casing for SVGAElement.target from the bindings generator
by using [ImplementedAs=svgTarget] for that attribute.

R=haraken

Review URL: https://chromiumcodereview.appspot.com/19256015

git-svn-id: svn://svn.chromium.org/blink/trunk@154309 bbb929c8-8fbe-4397-9dbb-9b2b20218538