Implement Physical Web listener registration

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