Update from https://crrev.com/317530

base/process/process_win.cc

Index: base/process/process_win.cc
diff --git a/base/process/process_win.cc b/base/process/process_win.cc
index 4e600f944db2063a6e97c0bd94c683a6e92471fd..8e5360b385603bd64cecf02e663123329a84381d 100644
--- a/base/process/process_win.cc
+++ b/base/process/process_win.cc
@@ -6,6 +6,7 @@
 
 #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"
 
@@ -165,7 +166,21 @@ bool Process::SetProcessBackgrounded(bool value) {
     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);