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

ios/web/web_thread_impl.cc

Index: ios/web/web_thread_impl.cc
diff --git a/ios/web/web_thread_impl.cc b/ios/web/web_thread_impl.cc
index 370295f259dca84215651729e5dc8d4c38e8f609..0f295c2ee6ea694a0e39f191baa28a5f2f668959 100644
--- a/ios/web/web_thread_impl.cc
+++ b/ios/web/web_thread_impl.cc
@@ -87,7 +87,10 @@ base::LazyInstance<WebThreadTaskRunners>::Leaky g_task_runners =
 
 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]));