Fix possible map::end() dereference in AppCacheUpdateJob triggered by a compromised renderer.

BUG=551044

Review URL: https://codereview.chromium.org/1418783005

Cr-Commit-Position: refs/heads/master@{#358815}
void AppCacheHost::AddObserver(Observer* observer) {
  observers_.AddObserver(observer);
}

void AppCacheHost::AddObserver(Observer* observer) {
  observers_.AddObserver(observer);
}
