Add TaskPriority as a parameter to SequencedWorkerPool in preparation for TaskScheduler experiment.

ios/web/web_thread_impl.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/web/web_thread_impl.h"
 
 #include <string>
 
 #include "base/atomicops.h"
 #include "base/bind.h"
@@ skipping 69 matching lines @@
   }
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runners[WebThread::ID_COUNT];
 };
 
 base::LazyInstance<WebThreadTaskRunners>::Leaky g_task_runners =
     LAZY_INSTANCE_INITIALIZER;
 
 struct WebThreadGlobals {
   WebThreadGlobals()
-      : blocking_pool(new base::SequencedWorkerPool(3, "WebBlocking")) {
+      : blocking_pool(
+            new base::SequencedWorkerPool(3,
+                                          "WebBlocking",
+                                          base::TaskPriority::USER_VISIBLE)) {
     memset(threads, 0, WebThread::ID_COUNT * sizeof(threads[0]));
     memset(thread_delegates, 0,
            WebThread::ID_COUNT * sizeof(thread_delegates[0]));
   }
 
   // This lock protects |threads|. Do not read or modify that array
   // without holding this lock. Do not block while holding this lock.
   base::Lock lock;
 
   // This array is protected by |lock|. The threads are not owned by this
@@ skipping 394 matching lines @@
   AtomicWord* storage =
       reinterpret_cast<AtomicWord*>(&globals.thread_delegates[identifier]);
   AtomicWord old_pointer = base::subtle::NoBarrier_AtomicExchange(
       storage, reinterpret_cast<AtomicWord>(delegate));
 
   // This catches registration when previously registered.
   DCHECK(!delegate || !old_pointer);
 }
 
 }  // namespace web