Handle RenderProcessHostImpl::SetBackgrounded on Windows

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index 68b50c70c6e1acbcdb54776df6cf335bf4d79410..ddd7a4b92bf1f739aefd6d1245bc542eaa0f597d 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -2207,17 +2207,27 @@ void RenderProcessHostImpl::SetBackgrounded(bool backgrounded) {
     return;
 #endif  // OS_WIN
 
+#if defined(OS_WIN)
+  // Same as below, but bound to an experiment (http://crbug.com/458594)
+  // initially on Windows. Enabled by default in the asbence of field trials to
+  // get coverage on the perf waterfall.
+  base::FieldTrial* trial =
+      base::FieldTrialList::Find("BackgroundRendererProcesses");
+  if (!trial || (trial->group_name() != "Disallow" &&
+                 trial->group_name() != "AllowBackgroundModeFromRenderer")) {
+    child_process_launcher_->SetProcessBackgrounded(backgrounded);
+  }
+#else
+  // Control the background state from the browser process, otherwise the task
+  // telling the renderer to "unbackground" itself may be preempted by other
+  // tasks executing at lowered priority ahead of it or simply by not being
+  // swiftly scheduled by the OS per the low process priority
+  // (http://crbug.com/398103).
+  child_process_launcher_->SetProcessBackgrounded(backgrounded);
+#endif  // OS_WIN
+
   // Notify the child process of background state.
   Send(new ChildProcessMsg_SetProcessBackgrounded(backgrounded));
-
-#if !defined(OS_WIN)
-  // Backgrounding may require elevated privileges not available to renderer
-  // processes, so control backgrounding from the process host.
-
-  // Windows Vista+ has a fancy process backgrounding mode that can only be set
-  // from within the process.
-  child_process_launcher_->SetProcessBackgrounded(backgrounded);
-#endif  // !OS_WIN
 }
 
 void RenderProcessHostImpl::OnProcessLaunched() {