Enable FeatureList for the GPU process.

content/app/content_main_runner.cc

Index: content/app/content_main_runner.cc
diff --git a/content/app/content_main_runner.cc b/content/app/content_main_runner.cc
index abe4f13051c45db0ac11f31e7ef09b59fe3f618b..c122ad8be12552f4c39c81b346847f9f77ad546c 100644
--- a/content/app/content_main_runner.cc
+++ b/content/app/content_main_runner.cc
@@ -15,15 +15,18 @@
 #include "base/allocator/allocator_check.h"
 #include "base/allocator/allocator_extension.h"
 #include "base/at_exit.h"
+#include "base/base_switches.h"
 #include "base/command_line.h"
 #include "base/debug/debugger.h"
 #include "base/debug/stack_trace.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/i18n/icu_util.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/path_service.h"
@@ -729,6 +732,32 @@ class ContentMainRunnerImpl : public ContentMainRunner {
     std::string process_type =
         command_line.GetSwitchValueASCII(switches::kProcessType);
 
+    // TODO(asvitkine): This logic should run for all child processes.

[Alexei Svitkine (slow), 2016/04/28 15:23:33]

Thanks! You can add 563705 to the BUG= line of this CL.

[erikchen, 2016/04/29 17:45:04]

Done.

+    if (process_type == switches::kGpuProcess ||
+        process_type == switches::kUtilityProcess ||
+        process_type == switches::kPpapiPluginProcess) {
+      // Initialize statistical testing infrastructure.  We set the entropy
+      // provider to NULL to disallow non-browser processes from creating their
+      // own one-time randomized trials; they should be created in the browser
+      // process.
+      base::FieldTrialList field_trial_list(NULL);

[Alexei Svitkine (slow), 2016/04/28 15:23:33]

This is not correct. The FieldTrialList should stay alive for the duration of
the process.

In this case, if you put it outside of the loop, it should be fine since this
calls RunNamedProcessTypeMain() and so the lifetime of FieldTrialList will
outlive whatever else the process does.

You can make it a unique_ptr above the loop and create the object in the loop.

[erikchen, 2016/04/29 17:45:04]

You keep talking about "the loop". Are you referring to the loop in
RunNamedProcessTypeMain()? I moved the object constructor to line 739, where it
should stay alive until RunNamedProcessTypeMain() returns. I prefer to run that
code as early as possible.

[Alexei Svitkine (slow), 2016/04/29 18:06:31]

Woops, when I said "loop" I meant "if". Sorry for the confusion.

+
+      // Ensure any field trials in browser are reflected into the child
+      // process.
+      if (command_line.HasSwitch(switches::kForceFieldTrials)) {

[Alexei Svitkine (slow), 2016/04/28 15:23:33]

It would be good to check whether we propagate this flag when launching child
processes. Not sure if you've done that already.

[erikchen, 2016/04/29 17:45:04]

We weren't propagating it. I added it to
ChildProcessHostImpl::CopyEnableDisableFeatureFlags.

+        bool result = base::FieldTrialList::CreateTrialsFromString(
+            command_line.GetSwitchValueASCII(switches::kForceFieldTrials),
+            std::set<std::string>());
+        DCHECK(result);
+      }
+
+      std::unique_ptr<base::FeatureList> feature_list(new base::FeatureList);
+      feature_list->InitializeFromCommandLine(
+          command_line.GetSwitchValueASCII(switches::kEnableFeatures),
+          command_line.GetSwitchValueASCII(switches::kDisableFeatures));
+      base::FeatureList::SetInstance(std::move(feature_list));
+    }
+
     base::HistogramBase::EnableActivityReportHistogram(process_type);
 
     MainFunctionParams main_params(command_line);