[StorageBuckets] Remove redundant StorageKey data in CacheStorageImpl

Introduced here: https://crrev.com/c/3351085

This change removes storing StorageKey twice inside
CacheStorageImpl, storage_key_ and bucket_.storage_key,
and adds a DCHECK to ensure that a bucket exists before
returning a CacheStorage object. They should always hold
the same value, therefore removing the data redundancy for
simplification.

Change-Id: Ic98975ba4480afe7422f66813f41d578eeb0a805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3373706
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/main@{#957604}