Currently, worker creation message for SharedWorker is synchronous because the ctor of SharedWorker needs to throw an exception in the case where worker creation fails. This behavior was defined by the HTML spec, but it was revised and now there is no case to thrown an exception (see  issue 538914 ). Therefore, we no longer have to make it synchronous.

This was pointed out by horo@:
https://codereview.chromium.org/2600113003/#msg40