Drop [LegacyImplementedInBaseClass] from SVGTests IDL interface

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

R=haraken, pdr
NOTRY=true
BUG=344224

Review URL: https://codereview.chromium.org/168973002

git-svn-id: svn://svn.chromium.org/blink/trunk@167251 bbb929c8-8fbe-4397-9dbb-9b2b20218538