IDL compiler: Implement OverloadGroup.index

Bug: 839389
Change-Id: I460a9d7fa5f9f0e0c25eee3a75b0813524ec937a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932148
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718558}

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

@@ -78,6 +78,9 @@ class OverloadGroup(WithIdentifier):
     def __len__(self):
         return len(self._functions)
 
+    def index(self, value):
+        return self._functions.index(value)
+
     @property
     def is_static(self):
         return self._is_static