ServiceWorker: Notify observers when the storage gets recovered from corruption state
This introduces OnStorageWiped event in ServiceWorkerContextObserver. The event
is notified when the storage gets recovered from the corruption state.
Note: OnRegistrationDelete events are not delivered in this case because we
cannot retrieve registration data to be deleted from the corrupted database.
BUG=438075
TEST=content_unittests --gtest_filter=ServiceWorkerContextTest.*
Review URL: https://codereview.chromium.org/
772333005
Cr-Commit-Position: refs/heads/master@{#307414}