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

components/physical_web/data_source/BUILD.gn

Index: components/physical_web/data_source/BUILD.gn
diff --git a/components/physical_web/data_source/BUILD.gn b/components/physical_web/data_source/BUILD.gn
index a0dffd5201f9b753c538a1beadda981e4ef39752..0d8944f4865fdc1b79b58ad5524c3c92c764ada3 100644
--- a/components/physical_web/data_source/BUILD.gn
+++ b/components/physical_web/data_source/BUILD.gn
@@ -16,6 +16,18 @@ source_set("data_source") {
   ]
 }
 
+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 = [