IDL compiler: Support indexing access to OverloadGroup

Bug: 839389
Change-Id: Ia31c0b27ee034e697551d170a961b131cf8415e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939989Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719596}

third_party/blink/renderer/bindings/scripts/web_idl/function_like.py

@@ -142,6 +142,9 @@ class OverloadGroup(WithIdentifier):
             function.set_overload_group(self)
         self._is_static = functions[0].is_static
 
+    def __getitem__(self, index):
+        return self._functions[index]
+
     def __iter__(self):
         return iter(self._functions)