AppCache: Use WeakPtr<> to fix a potential uaf bug.

BUG=554908

Review URL: https://codereview.chromium.org/1441683004

Cr-Commit-Position: refs/heads/master@{#359930}
void AppCacheDispatcherHost::OnUnregisterHost(int host_id) {
  if (appcache_service_.get()) {
    if (!backend_impl_.UnregisterHost(host_id)) {
      bad_message::ReceivedBadMessage(this, bad_message::ACDH_UNREGISTER);
    }
  }
}

void AppCacheDispatcherHost::OnUnregisterHost(int host_id) {
  if (appcache_service_.get()) {
    if (!backend_impl_.UnregisterHost(host_id)) {
      bad_message::ReceivedBadMessage(this, bad_message::ACDH_UNREGISTER);
    }
  }
}
