See e.g. crash/a43ddc939316c9be.

void QuotaTracker::GetDatabaseSizeAndSpaceAvailableToOrigin(
    SecurityOrigin* origin,
    const String& database_name,
    unsigned long long* database_size,
    unsigned long long* space_available) {
  // Extra scope to unlock prior to potentially calling Platform.
  {
    MutexLocker lock_data(data_guard_);
    DCHECK(database_sizes_.Contains(origin->ToRawString()));   <<< HERE
    HashMap<String, SizeMap>::const_iterator it =
        database_sizes_.find(origin->ToRawString());
    DCHECK(it->value.Contains(database_name));
    *database_size = it->value.at(database_name);
  }

  // The embedder hasn't pushed this value to us, so we pull it as needed.
  *space_available = Platform::Current()->DatabaseGetSpaceAvailableForOrigin(
      WebSecurityOrigin(origin));
}