Update from https://crrev.com/317530

base/process/process_win.cc

 // Copyright (c) 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.
 
 #include "base/process/process.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/metrics/field_trial.h"
 #include "base/process/kill.h"
 #include "base/win/windows_version.h"
 
 namespace {
 
 DWORD kBasicProcessAccess =
   PROCESS_TERMINATE | PROCESS_QUERY_INFORMATION | SYNCHRONIZE;
 
 } // namespace
 
@@ skipping 139 matching lines @@
 bool Process::SetProcessBackgrounded(bool value) {
   DCHECK(IsValid());
   // Vista and above introduce a real background mode, which not only
   // sets the priority class on the threads but also on the IO generated
   // by it. Unfortunately it can only be set for the calling process.
   DWORD priority;
   if ((base::win::GetVersion() >= base::win::VERSION_VISTA) && (is_current())) {
     priority = value ? PROCESS_MODE_BACKGROUND_BEGIN :
                        PROCESS_MODE_BACKGROUND_END;
   } else {
-    priority = value ? BELOW_NORMAL_PRIORITY_CLASS : NORMAL_PRIORITY_CLASS;
+    // Experiment (http://crbug.com/458594) with using IDLE_PRIORITY_CLASS as a
+    // background priority for background renderers (this code path is
+    // technically for more than just the renderers but they're the only use
+    // case in practice and experimenting here direclty is thus easier -- plus
+    // it doesn't really hurt as above we already state our intent of using
+    // PROCESS_MODE_BACKGROUND_BEGIN if available which is essentially
+    // IDLE_PRIORITY_CLASS plus lowered IO priority). Enabled by default in the
+    // asbence of field trials to get coverage on the perf waterfall.
+    DWORD background_priority = IDLE_PRIORITY_CLASS;
+    base::FieldTrial* trial =
+        base::FieldTrialList::Find("BackgroundRendererProcesses");
+    if (trial && trial->group_name() == "AllowBelowNormalFromBrowser")
+      background_priority = BELOW_NORMAL_PRIORITY_CLASS;
+
+    priority = value ? background_priority : NORMAL_PRIORITY_CLASS;
   }
 
   return (::SetPriorityClass(Handle(), priority) != 0);
 }
 
 int Process::GetPriority() const {
   DCHECK(IsValid());
   return ::GetPriorityClass(Handle());
 }
 
 }  // namespace base