Migrate ArcBridgeService::Observer and ArcSession::Observer.

components/arc/arc_session_observer.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 "components/arc/arc_session_observer.h"
+
+namespace arc {
+
+std::ostream& operator<<(std::ostream& os,
+                         ArcSessionObserver::StopReason reason) {
+  switch (reason) {
+#define CASE_IMPL(val)                      \
+  case ArcSessionObserver::StopReason::val: \
+    return os << #val
+
+    CASE_IMPL(SHUTDOWN);
+    CASE_IMPL(GENERIC_BOOT_FAILURE);
+    CASE_IMPL(LOW_DISK_SPACE);
+    CASE_IMPL(CRASH);
+#undef CASE_IMPL
+  }
+
+  // In case of unexpected value, output the int value.
+  return os << "StopReason(" << static_cast<int>(reason) << ")";
+}
+
+}  // namespace arc