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::OnSetSpawningHostId(
    int host_id, int spawning_host_id) {
  if (appcache_service_.get()) {
    if (!backend_impl_.SetSpawningHostId(host_id, spawning_host_id))
      bad_message::ReceivedBadMessage(this, bad_message::ACDH_SET_SPAWNING);
  }
}

void AppCacheDispatcherHost::OnSetSpawningHostId(
    int host_id, int spawning_host_id) {
  if (appcache_service_.get()) {
    if (!backend_impl_.SetSpawningHostId(host_id, spawning_host_id))
      bad_message::ReceivedBadMessage(this, bad_message::ACDH_SET_SPAWNING);
  }
}
