Add webusb notification UI code

components/webusb/webusb_browser_client.cc

+// Copyright 2015 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/webusb/webusb_browser_client.h"
+
+namespace webusb {
+
+namespace {
+
+WebUsbBrowserClient* webusb_browser_client = nullptr;
+
+}  // namespace
+
+WebUsbBrowserClient* WebUsbBrowserClient::Get() {
+  if (!webusb_browser_client ||
+      !webusb_browser_client->WebUsbNotificationEnabled()) {

[Reilly Grant (use Gerrit), 2015/08/20 17:02:45]

This feels weird. Why not just not Set() the browser client if the flag isn't
set?

[juncai, 2015/08/21 05:16:04]

Done.

+    return nullptr;
+  }
+  return webusb_browser_client;
+}
+
+void WebUsbBrowserClient::Set(WebUsbBrowserClient* client) {
+  webusb_browser_client = client;
+}
+
+}  // namespace webusb