Add CHECK for bindings_system_ to ServiceWorkerData::Init

In crrev.com/c/4902564, multiple CHECKs and if conditions were
introduced to ServiceWorkerData, and some of them were removed in
crrev.com/c/5646620. Now we can ensure that `bindings_system_` always
exists in `Init()`.

Bug: 389971360
Change-Id: Ibc5cc6c7ad24985867c75102b5c58a02f4e405f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6176646
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1410107}