Add LegacyImplementedInBaseClass check to interface_dependency_resolver.py

Christophe added a new extended attribute last week that changes the parsing
(JSON) slightly (of Window.idl and WorkerGlobalScope.idl).
Just noticed this, so adding to Python.

Reference:
Issue 19675008: Move btoa() / atob() implementation out of DOMWindow
https://chromiumcodereview.appspot.com/19675008

BUG=261226

Review URL: https://chromiumcodereview.appspot.com/19693021

git-svn-id: svn://svn.chromium.org/blink/trunk@154895 bbb929c8-8fbe-4397-9dbb-9b2b20218538