Refactor out a new class for syncing field trials in child processes.

chrome/common/variations/child_process_field_trial_syncer.cc

Index: chrome/common/variations/child_process_field_trial_syncer.cc
diff --git a/chrome/common/variations/child_process_field_trial_syncer.cc b/chrome/common/variations/child_process_field_trial_syncer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9a0b309481e4e4656296b4c3184e328a0bdec03e
--- /dev/null
+++ b/chrome/common/variations/child_process_field_trial_syncer.cc
@@ -0,0 +1,67 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/common/variations/child_process_field_trial_syncer.h"
+
+#include <set>
+#include <utility>
+
+#include "base/base_switches.h"
+#include "base/command_line.h"
+#include "chrome/common/render_messages.h"
+#include "components/variations/variations_util.h"
+
+namespace chrome_variations {
+
+ChildProcessFieldTrialSyncer::ChildProcessFieldTrialSyncer(
+    IPC::Sender* ipc_sender) : ipc_sender_(ipc_sender) {}
+
+ChildProcessFieldTrialSyncer::~ChildProcessFieldTrialSyncer() {}
+
+void ChildProcessFieldTrialSyncer::InitFieldTrialObserving(
+    const base::CommandLine& command_line) {
+  // Set up initial set of crash dump data for field trials in this process.
+  variations::SetVariationListCrashKeys();
+
+  // Listen for field trial activations to report them to the browser.
+  base::FieldTrialList::AddObserver(this);
+
+  // Some field trials may have been activated before this point. Notify the
+  // browser of these activations now. To detect these, take the set difference
+  // of currently active trials with the initially active trials.
+  base::FieldTrial::ActiveGroups initially_active_trials;
+  base::FieldTrialList::GetActiveFieldTrialGroupsFromString(
+      command_line.GetSwitchValueASCII(switches::kForceFieldTrials),
+      &initially_active_trials);
+  std::set<std::string> initially_active_trials_set;
+  for (const auto& entry : initially_active_trials) {
+    initially_active_trials_set.insert(std::move(entry.trial_name));
+  }
+
+  base::FieldTrial::ActiveGroups current_active_trials;
+  base::FieldTrialList::GetActiveFieldTrialGroups(&current_active_trials);
+  for (const auto& trial : current_active_trials) {
+    if (!ContainsKey(initially_active_trials_set, trial.trial_name))
+      OnFieldTrialGroupFinalized(trial.trial_name, trial.group_name);
+  }
+}
+
+void ChildProcessFieldTrialSyncer::OnSetFieldTrialGroup(
+    const std::string& trial_name,
+    const std::string& group_name) {
+  base::FieldTrial* trial =
+      base::FieldTrialList::CreateFieldTrial(trial_name, group_name);
+  // Ensure the trial is marked as "used" by calling group() on it if it is
+  // marked as activated.
+  trial->group();
+  variations::SetVariationListCrashKeys();
+}
+
+void ChildProcessFieldTrialSyncer::OnFieldTrialGroupFinalized(
+    const std::string& trial_name,
+    const std::string& group_name) {
+  ipc_sender_->Send(new ChromeViewHostMsg_FieldTrialActivated(trial_name));
+}
+
+}  // namespace chrome_variations