Before this change, .ready method was waiting for the change of the
state of the regisration's active worker without being synchronized with
the steps that set the corresponding ServiceWorkerRegistration object's
active ServiceWorker object's state.

This changes both .ready and Activate such that .ready tries to resolve
the ready promise, if pending, and just returns. And Activate resolves
pending ready promises after updating active ServiceWorker object's
state.

Fixes #1011

…e main thread