VR: Use ToolbarModel to drive VR URL bar state.

chrome/browser/android/vr_shell/toolbar_helper.h

+// 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.
+
+#ifndef CHROME_BROWSER_ANDROID_VR_SHELL_TOOLBAR_HELPER_H_
+#define CHROME_BROWSER_ANDROID_VR_SHELL_TOOLBAR_HELPER_H_
+
+#include "chrome/browser/android/vr_shell/toolbar_state.h"
+#include "chrome/browser/android/vr_shell/ui_interface.h"
+
+class ToolbarModel;
+class ToolbarModelDelegate;
+
+namespace vr_shell {
+
+class UiInterface;
+
+// This class houses an instance of ToolbarModel, and when asked, passes a

[mthiesse, 2017/06/28 00:27:08]

nit: Clarify or remove "when asked"? 

Maybe "This class houses an instance of ToolbarModel, and updates it when
requested, passing a snapshot of the toolbar state to the UI when necessary."

[cjgrant, 2017/06/28 15:14:34]

Done.  Your wording, except we don't update the ToolbarModel, we query it.

+// snapshot of the toolbar state to the UI.
+class ToolbarHelper {
+ public:
+  ToolbarHelper(UiInterface* ui, ToolbarModelDelegate* delegate);
+  virtual ~ToolbarHelper();
+
+  // Poll ToolbarModel and post an update to the UI if state has changed.
+  void Update();
+
+ private:
+  UiInterface* ui_;
+  std::unique_ptr<ToolbarModel> toolbar_model_;
+  ToolbarState current_state_;
+};
+
+}  // namespace vr_shell
+
+#endif  // CHROME_BROWSER_ANDROID_VR_SHELL_TOOLBAR_HELPER_H_