Create skeleton for the Prefetching store and initial pipeline step.

components/offline_pages/core/prefetch/BUILD.gn

Index: components/offline_pages/core/prefetch/BUILD.gn
diff --git a/components/offline_pages/core/prefetch/BUILD.gn b/components/offline_pages/core/prefetch/BUILD.gn
index ae365d2bd5aa601ac8c0f057f4d28c3177fdf3f7..5ab72e6fc981ac4cbed3ac3d00fb8bb903976081 100644
--- a/components/offline_pages/core/prefetch/BUILD.gn
+++ b/components/offline_pages/core/prefetch/BUILD.gn
@@ -9,6 +9,8 @@ import("//third_party/protobuf/proto_library.gni")
 
 static_library("prefetch") {
   sources = [
+    "add_unique_urls_task.cc",
+    "add_unique_urls_task.h",
     "generate_page_bundle_request.cc",
     "generate_page_bundle_request.h",
     "get_operation_request.cc",
@@ -25,6 +27,9 @@ static_library("prefetch") {
     "prefetch_service.h",
     "prefetch_service_impl.cc",
     "prefetch_service_impl.h",
+    "prefetch_store.h",
+    "prefetch_test_store.cc",
+    "prefetch_test_store.h",
     "prefetch_types.cc",
     "prefetch_types.h",
   ]