Remove support for AllowBackgroundModeFromRenderer group in BackgroundRendererProcesses experiment.

This group was only intended to be ran on early channels to show that we are tracking the right stats and ensure that the latest background mode experiment isn't bringing back the regressions that mode had previously brought (see issue 381820 and issue 398103).

The analysis is complete and support for it in the client is no longer required (already ramped down server-side).

(this is essentially a partial revert of https://codereview.chromium.org/926663002)

BUG=459600

Review URL: https://codereview.chromium.org/1089873002

Cr-Commit-Position: refs/heads/master@{#325445}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2250,10 +2250,8 @@ void RenderProcessHostImpl::SetBackgrounded(bool backgrounded) {
   // get coverage on the perf waterfall.
   base::FieldTrial* trial =
       base::FieldTrialList::Find("BackgroundRendererProcesses");
-  if (!trial || (trial->group_name() != "Disallow" &&
-                 trial->group_name() != "AllowBackgroundModeFromRenderer")) {
+  if (!trial || trial->group_name() != "Disallow")
     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

content/child/child_thread_impl.cc

@@ -689,23 +689,6 @@ void ChildThreadImpl::OnProcessBackgrounded(bool background) {
   if (background)
     timer_slack = base::TIMER_SLACK_MAXIMUM;
   base::MessageLoop::current()->SetTimerSlack(timer_slack);
-
-#ifdef OS_WIN
-  // Windows Vista+ has a fancy process backgrounding mode that can only be set
-  // from within the process. This used to be how chrome set its renderers into
-  // background mode on Windows but was removed due to http://crbug.com/398103.
-  // As we experiment with bringing back some other form of background mode for
-  // hidden renderers, add a bucket to allow us to trigger this undesired method
-  // of setting background state in order to confirm that the metrics which were
-  // added to prevent regressions on the aforementioned issue indeed catch such
-  // regressions and are thus a reliable way to confirm that our latest proposal
-  // doesn't cause such issues. TODO(gab): Remove this once the experiment is
-  // over (http://crbug.com/458594).
-  base::FieldTrial* trial =
-      base::FieldTrialList::Find("BackgroundRendererProcesses");
-  if (trial && trial->group_name() == "AllowBackgroundModeFromRenderer")
-    base::Process::Current().SetProcessBackgrounded(background);
-#endif  // OS_WIN
 }
 
 }  // namespace content