Drop [LegacyImplementedInBaseClass] from SVGFilterPrimitiveStandardAttributes IDL interface

Drop [LegacyImplementedInBaseClass] from SVGFilterPrimitiveStandardAttributes
IDL interface by refactoring the code a little bit.

R=haraken, pdr
BUG=344224

Review URL: https://codereview.chromium.org/168983002

git-svn-id: svn://svn.chromium.org/blink/trunk@167254 bbb929c8-8fbe-4397-9dbb-9b2b20218538