Fix -Wdefaulted-function-deleted warning

../../extensions/browser/service_worker_task_queue.cc:77:21: note: move
assignment operator of 'WaitingDidStartWorkerTask' is implicitly deleted
because field 'extension_id' has no move assignment operator
  const ExtensionId extension_id;
                    ^

Bug: 890307
Change-Id: Ifb5eac24bd150c9a20d170f0ef084334abfb6601
Reviewed-on: https://chromium-review.googlesource.com/c/1256807Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596139}

extensions/browser/service_worker_task_queue.cc

@@ -70,8 +70,6 @@ struct ServiceWorkerTaskQueue::WaitingDidStartWorkerTask {
         thread_id(thread_id) {}
 
   WaitingDidStartWorkerTask(WaitingDidStartWorkerTask&& other) = default;
-  WaitingDidStartWorkerTask& operator=(WaitingDidStartWorkerTask&& other) =
-      default;
 
   LazyContextTaskQueue::PendingTask task;
   const ExtensionId extension_id;