Add webusb notification UI unit tests

components/webusb/BUILD.gn

 # 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.
 
 static_library("webusb") {
   sources = [
     "webusb_browser_client.h",
     "webusb_detector.cc",
     "webusb_detector.h",
   ]
 
   deps = [
     "//base",
     "//device/core",
     "//device/usb",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "webusb_detector_unittest.cc",
+  ]
+
+  deps = [
+    ":webusb",
+    "//device/usb",
+    "//device/usb:mocks",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}