If we specify arguments to [CustomConstructor] extended attribute (similarly to [Constructor]), then we no longer need the [ConstructorParameters] extended attribute to determine the constructor length. This is less error prone, leads to more readable IDL, and more consistent with [Constructor] extended attribute. This also reduces the number of WebKit specific IDL attributes, which is always good.
FYI, I already did the same thing in Blink: https://src.chromium.org/viewvc/blink?view=rev&revision=148663
Created attachment 202082 [details] Patch
Comment on attachment 202082 [details] Patch Looks great!
Comment on attachment 202082 [details] Patch Clearing flags on attachment: 202082 Committed r150292: <http://trac.webkit.org/changeset/150292>
All reviewed patches have been landed. Closing bug.
Updated WebKit IDL wiki accordingly: http://trac.webkit.org/wiki/WebKitIDL?action=diff&version=117&old_version=116