Mark anonymous indexed getters argument as mandatory

Mark anonymous indexed getters argument as mandatory since there is no way
for the caller to omit the index. The generated bindings are exactly the
same and therefore there is no behavior change.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23519003

git-svn-id: svn://svn.chromium.org/blink/trunk@156998 bbb929c8-8fbe-4397-9dbb-9b2b20218538