Simplify RadioNodeList's anonymous indexed getter

Make RadioNodeList's anonymous indexed getter argument mandatory and drop
the [IsIndex] IDL extended attribute. 'optional' and [IsIndex] have no
effect on the generated code of anonymous indexed getters:
- There is no way for the caller to omit the index
- The generated code uses ObjectTemplate::SetIndexedPropertyHandler() and
the IndexedPropertyGetterCallback takes a uint32_t in argument.

There is no web-exposed behavior change, the generated bindings code is
identical.

R=haraken
BUG=237739

Review URL: https://chromiumcodereview.appspot.com/21891009

git-svn-id: svn://svn.chromium.org/blink/trunk@155535 bbb929c8-8fbe-4397-9dbb-9b2b20218538