Extensions: Checking for failure when reading settings from db.

Previously SettingsStorageQuotaEnforcer::CalculateUsage() made one
attempt to restore the db, but assumed that a second call to Get()
would always return a valid settings dictionary.

BUG=554657

Review URL: https://codereview.chromium.org/1434133002

Cr-Commit-Position: refs/heads/master@{#359866}
2 files changed