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

chrome/common/variations/child_process_field_trial_syncer.cc

+// 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 "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;

[Lei Zhang, 2016/06/01 20:52:51]

IWYU: <set>

[Alexei Svitkine (slow), 2016/06/01 21:00:39]

Done.

+  for (const auto& entry : initially_active_trials) {
+    initially_active_trials_set.insert(std::move(entry.trial_name));

[Lei Zhang, 2016/06/01 20:52:51]

IYWU: <utility>

[Alexei Svitkine (slow), 2016/06/01 21:00:38]

Done.

+  }
+
+  base::FieldTrial::ActiveGroups current_active_trials;
+  base::FieldTrialList::GetActiveFieldTrialGroups(&current_active_trials);
+  for (const auto& trial : current_active_trials) {

[Lei Zhang, 2016/06/01 20:52:51]

Side question as this is just refactoring existing code...

How many times do we typically call OnFieldTrialGroupFinalized() which then
sends an IPC message? Can ChromeViewHostMsg_FieldTrialActivated take a
std::set/vector to reduce the number of messages?

[Alexei Svitkine (slow), 2016/06/01 21:00:39]

The initial set is actually sent on the command-line at process start up as a
list.

The entries sent individually via IPC are sent later as they get activated, so
it's not really possible to batch them (without e.g. delaying them and having
some threshold for batching them). But I don't think the latter is frequent
enough to warrant this. (i.e. individual activations happen rarely and depend on
usage)

+    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