[Background Sync] Remove the TODO about waiting for an installing worker

The new version of the spec removes the clause about waiting for
installing workers since that can easily cause deadlocks.

Bug: 542437
Change-Id: I4192787248e3d147ec001516f86aa17ea361752f
Reviewed-on: https://chromium-review.googlesource.com/c/1323717Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606418}

third_party/blink/renderer/modules/background_sync/sync_manager.cc

@@ -25,8 +25,6 @@ SyncManager::SyncManager(ServiceWorkerRegistration* registration)
 
 ScriptPromise SyncManager::registerFunction(ScriptState* script_state,
                                             const String& tag) {
-  // TODO(jkarlin): Wait for the registration to become active instead of
-  // rejecting. See crbug.com/542437.
   if (!registration_->active()) {
     return ScriptPromise::RejectWithDOMException(
         script_state,