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

ios/chrome/browser/net/image_fetcher_unittest.mm

 // Copyright 2011 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.
 
 #import "ios/chrome/browser/net/image_fetcher.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/ios/ios_util.h"
 #include "base/mac/scoped_block.h"
@@ skipping 55 matching lines @@
     "HTTP/1.1 200 OK\0Content-type: image/webp\0\0";
 
 }  // namespace
 
 // TODO(ios): remove the static_cast<UIImage*>(nil) once all the bots have
 // Xcode 6.0 or later installed, http://crbug.com/440857
 
 class ImageFetcherTest : public PlatformTest {
  protected:
   ImageFetcherTest()
-      : pool_(new base::SequencedWorkerPool(1, "TestPool")),
+      : pool_(new base::SequencedWorkerPool(1,
+                                            "TestPool",
+                                            base::TaskPriority::USER_VISIBLE)),
         image_fetcher_(new ImageFetcher(pool_)),
         result_(nil),
         called_(false) {
     callback_.reset(
         [^(const GURL& original_url, int http_response_code, NSData* data) {
             result_ = [UIImage imageWithData:data];
             called_ = true;
         } copy]);
     image_fetcher_->SetRequestContextGetter(
         new net::TestURLRequestContextGetter(
@@ skipping 97 matching lines @@
   EXPECT_EQ(static_cast<UIImage*>(nil), result_);
   EXPECT_FALSE(called_);
 }
 
 TEST_F(ImageFetcherTest, TestCallbacksNotCalledDuringDeletion) {
   image_fetcher_->StartDownload(GURL(kTestUrl), callback_);
   image_fetcher_.reset();
   EXPECT_FALSE(called_);
 }