Use ToResolvedPromise<> instead of ScriptPromise::Cast

Change-Id: I6e70c42cecba1c15f4421308cfe7457c374dd16b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5332836


Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267360}