[PhysicalWeb] Add fake data source and helpers for testing.

components/physical_web/data_source/BUILD.gn

 # Copyright 2016 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.
 
 source_set("data_source") {
   sources = [
     "physical_web_data_source.cc",
     "physical_web_data_source.h",
     "physical_web_data_source_impl.cc",
     "physical_web_data_source_impl.h",
     "physical_web_listener.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
+source_set("test_support") {
+  testonly = true
+  sources = [
+    "fake_physical_web_data_source.cc",
+    "fake_physical_web_data_source.h",
+  ]
+  deps = [
+    ":data_source",
+    "//base",
+  ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [
     "physical_web_data_source_impl_unittest.cc",
   ]
 
   deps = [
     ":data_source",
     "//base",
     "//testing/gtest",
   ]
 }