Drop [LegacyImplementedInBaseClass] from SVGURIReference IDL interface

Drop [LegacyImplementedInBaseClass] from SVGURIReference IDL interface by
refactoring the code. Also move the SVGURIReference code from subclasses to
SVGURIReference to avoid code duplication.

R=haraken, pdr
BUG=344224

Review URL: https://codereview.chromium.org/169103002

git-svn-id: svn://svn.chromium.org/blink/trunk@167255 bbb929c8-8fbe-4397-9dbb-9b2b20218538