[Sync] Split UserEventService into interface and impl, add fake impl, add unit tests.

components/sync/user_events/fake_user_event_service.cc

+// Copyright 2017 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 "components/sync/user_events/fake_user_event_service.h"
+
+using sync_pb::UserEventSpecifics;
+
+namespace syncer {
+
+FakeUserEventService::FakeUserEventService() {}
+
+FakeUserEventService::~FakeUserEventService() {}
+
+void FakeUserEventService::RecordUserEvent(
+    std::unique_ptr<UserEventSpecifics> specifics) {
+  DCHECK(specifics);
+  RecordUserEvent(*specifics);
+}
+
+void FakeUserEventService::RecordUserEvent(
+    const UserEventSpecifics& specifics) {
+  recorded_user_events_.push_back(specifics);
+}
+
+base::WeakPtr<ModelTypeSyncBridge> FakeUserEventService::GetSyncBridge() {
+  return base::WeakPtr<ModelTypeSyncBridge>();
+}
+
+void FakeUserEventService::RegisterDependentFieldTrial(
+    const std::string& trial_name,
+    UserEventSpecifics::EventCase event_case) {
+  registered_dependent_field_trials_[trial_name].insert(event_case);
+}
+
+const std::vector<UserEventSpecifics>&
+FakeUserEventService::GetRecordedUserEvents() const {
+  return recorded_user_events_;
+}
+
+const std::map<std::string, std::set<sync_pb::UserEventSpecifics::EventCase>>&
+FakeUserEventService::GetRegisteredDependentFieldTrials() const {
+  return registered_dependent_field_trials_;
+}
+
+}  // namespace syncer