Add support for backgrounding processes on the Mac

content/browser/child_process_launcher.cc

Index: content/browser/child_process_launcher.cc
diff --git a/content/browser/child_process_launcher.cc b/content/browser/child_process_launcher.cc
index e5c93ee3562bbecd378d96c671d77c4fe219c39e..53499b99170e99b3a5feade3b8b6a8c79415f92b 100644
--- a/content/browser/child_process_launcher.cc
+++ b/content/browser/child_process_launcher.cc
@@ -12,6 +12,7 @@
 #include "base/files/scoped_file.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/process/process.h"
 #include "base/synchronization/lock.h"
@@ -503,7 +504,17 @@ void ChildProcessLauncher::Context::Terminate() {
 void ChildProcessLauncher::Context::SetProcessBackgroundedInternal(
     base::Process process,
     bool background) {
+#if defined(OS_MACOSX)
+  base::FieldTrial* trial =
+      base::FieldTrialList::Find("BackgroundMacRendererProcesses");

[Alexei Svitkine (slow), 2015/03/11 15:30:54]

Nit: You can just use FindFullName() and then the logic below becomes simpler.
(It will return "" if no trial).

[shrike, 2015/03/11 21:38:21]

Done.

+  if (trial && trial->group_name() == "On") {

[gab, 2015/03/10 12:33:02]

I would suggest taking this path by default in the absence of field trials,
i.e.:
if (!trial || trial->group_name() == "On") {...

This will make this the default behavior on bots (in particular perf bots).

The only downside is that you'll have to make sure to have a Finch config
initially explicitly disabling it on Beta/Stable.

[shrike, 2015/03/10 18:00:33]

OK. Will Finch flags be available soon enough after startup so that they'll
exist before any render processes get backgrounded?

[gab, 2015/03/10 19:43:50]

Yes, Finch is brought up before creating any threads/processes if I'm no
mistaken.

@asvitkine to confirm

[Alexei Svitkine (slow), 2015/03/11 15:30:54]

Correct.

+    MachBroker* broker = MachBroker::GetInstance();
+    process.SetProcessBackgrounded(broker->TaskForPid(process.Pid()),
+                                   background);
+  }
+#else
   process.SetProcessBackgrounded(background);
+#endif  // defined(OS_MACOSX)
 #if defined(OS_ANDROID)
   SetChildProcessInForeground(process.Handle(), !background);
 #endif