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::GetStatusCallback(
    AppCacheStatus status, void* param) {
  IPC::Message* reply_msg = reinterpret_cast<IPC::Message*>(param);
  DCHECK_EQ(pending_reply_msg_.get(), reply_msg);
  AppCacheHostMsg_GetStatus::WriteReplyParams(reply_msg, status);
  Send(pending_reply_msg_.release());
}

void AppCacheDispatcherHost::GetStatusCallback(
    AppCacheStatus status, void* param) {
  IPC::Message* reply_msg = reinterpret_cast<IPC::Message*>(param);
  DCHECK_EQ(pending_reply_msg_.get(), reply_msg);
  AppCacheHostMsg_GetStatus::WriteReplyParams(reply_msg, status);
  Send(pending_reply_msg_.release());
}
