Add a row in the network tray to inform users to turn Bluetooth on to enable Tether.

ash/system/tray/tray_info_label.h

Index: ash/system/tray/tray_info_label.h
diff --git a/ash/system/tray/tray_info_label.h b/ash/system/tray/tray_info_label.h
new file mode 100644
index 0000000000000000000000000000000000000000..2267e945bd65ca351ef924faa74848f0ee4f3ab2
--- /dev/null
+++ b/ash/system/tray/tray_info_label.h
@@ -0,0 +1,57 @@
+// 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 ASH_SYSTEM_TRAY_TRAY_INFO_LABEL_H_
+#define ASH_SYSTEM_TRAY_TRAY_INFO_LABEL_H_
+
+#include "ash/system/tray/actionable_view.h"
+#include "ui/views/controls/label.h"
+
+namespace ash {
+
+// A view containing only a label, which is to be inserted as a
+// row within a system menu detailed view (e.g., the "Scanning for devices..."
+// message that can appear at the top of the Bluetooth detailed view).
+// TrayInfoLabel can be clickable; this property is configured by its delegate.
+class TrayInfoLabel : public ActionableView {
+ public:
+  // A delegate for determining whether or not a TrayInfoLabel is clickable, and
+  // handling actions when it is clicked.
+  class Delegate {
+   public:
+    virtual ~Delegate() {}
+    virtual void OnLabelClicked(int message_id) = 0;
+    virtual bool IsLabelClickable(int message_id) const = 0;
+  };
+
+  // |delegate| may be null, which results in a TrayInfoLabel which cannot be
+  // clicked.
+  TrayInfoLabel(Delegate* delegate, int message_id);
+  ~TrayInfoLabel() override{};
+
+  // Updates the TrayInfoLabel to display the message associated with
+  // |message_id|. This may update text styling if the delegate indicates that
+  // the TrayInfoLabel should be clickable.
+  void Update(int message_id);
+
+  // ActionableView:
+  bool PerformAction(const ui::Event& event) override;
+  void GetAccessibleNodeData(ui::AXNodeData* node_data) override;
+
+ private:
+  friend class TrayInfoLabelTest;
+
+  bool IsClickable();
+
+  views::Label* const label_;
+  int message_id_;
+
+  Delegate* delegate_;
+
+  DISALLOW_COPY_AND_ASSIGN(TrayInfoLabel);
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_TRAY_TRAY_INFO_LABEL_H_