Get rid of special casing for 'operator' from the bindings generator

Get rid of special casing for 'operator' from the bindings generator
by using [ImplementedAs=svgOperator] extended attribute for the
following attributes:
- SVGFECompositeElement.operator
- SVGFEMorphologyElement.operator

R=haraken

Review URL: https://chromiumcodereview.appspot.com/19368004

git-svn-id: svn://svn.chromium.org/blink/trunk@154341 bbb929c8-8fbe-4397-9dbb-9b2b20218538