Expose Physical Web metadata to native clients over JNI

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