Add a TaskPriority cap to the SWP redirection to experiment with no redirections at USER_BLOCKING.

components/task_scheduler_util/variations/browser_variations_util.cc

 // Copyright 2016 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 "components/task_scheduler_util/variations/browser_variations_util.h"
 
 #include <map>
 #include <string>
 #include <vector>
 
@@ skipping 118 matching lines @@
   std::map<std::string, std::string> variation_params;
   ::variations::GetVariationParams(kFieldTrialName, &variation_params);
 
   // TODO(gab): Remove this when http://crbug.com/622400 concludes.
   const auto sequenced_worker_pool_param =
       variation_params.find("RedirectSequencedWorkerPools");
   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kDisableBrowserTaskScheduler) &&
       sequenced_worker_pool_param != variation_params.end() &&
       sequenced_worker_pool_param->second == "true") {
-    base::SequencedWorkerPool::EnableWithRedirectionToTaskSchedulerForProcess();
+    // Check a variation that allows capping all redirections at USER_VISIBLE
+    // (no USER_BLOCKING) to observe the side-effects of multiple priority
+    // levels in the foreground IO pool.
+    const auto sequenced_worker_pool_cap_priority_param =
+        variation_params.find("CapSequencedWorkerPoolsAtUserVisible");
+
+    const base::TaskPriority max_task_priority =
+        sequenced_worker_pool_cap_priority_param != variation_params.end() &&
+        sequenced_worker_pool_cap_priority_param->second == "true"
+            ? base::TaskPriority::USER_VISIBLE
+            : base::TaskPriority::HIGHEST;
+
+    base::SequencedWorkerPool::EnableWithRedirectionToTaskSchedulerForProcess(
+        max_task_priority);
   } else {
     base::SequencedWorkerPool::EnableForProcess();
   }
 }
 
 }  // variations
 }  // namespace task_scheduler_util