VR: Factor transient timing out of UiSceneManager.

chrome/browser/android/vr_shell/ui_elements/transience_manager.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 "chrome/browser/android/vr_shell/ui_elements/transience_manager.h"
+
+namespace vr_shell {
+
+void TransienceManager::SetEnabled(bool enabled) {
+  if (enabled_ == enabled)
+    return;
+  enabled_ = enabled;
+  if (!enabled) {
+    element_->set_visible(false);
+    visibility_timer_.Stop();
+    return;

[bshe, 2017/06/27 14:57:09]

no need to return 

[cjgrant, 2017/06/28 19:00:55]

Done.

+  }
+}
+
+void TransienceManager::KickVisibility() {
+  if (enabled_) {
+    element_->set_visible(true);
+    StartTimer();
+  }
+}
+
+void TransienceManager::StartTimer() {
+  visibility_timer_.Start(
+      FROM_HERE, timeout_,
+      base::Bind(&TransienceManager::OnTimeout, base::Unretained(this)));
+}
+
+void TransienceManager::OnTimeout() {
+  element_->set_visible(false);
+}
+
+}  // namespace vr_shell