Clean up WKContentRuleListProvider callback

The `success` boolean in the callback was redundant with the `error`, so
it could be removed, leaving just the `error` parameter to be handled.
This simplifies the logic in both WKContentRuleListProvider and its
tests.

Bug: 418218469
Change-Id: I10b0638ebc6f4860523f043e71e95cf7ec004c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6602753
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Ron Sailer <rsailer@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1468160}