Add base persistence interfaces for Prefetching Offline Pages.

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 85d9fcba5f06856bec8809f4051056e683447ded..682b537bca997b9cd9b8ea59e7c08c99f6e22769 100644
--- a/components/offline_pages/core/prefetch/BUILD.gn
+++ b/components/offline_pages/core/prefetch/BUILD.gn
@@ -8,11 +8,14 @@ if (is_android) {
 
 static_library("prefetch") {
   sources = [
+    "prefetch_item.cc",
+    "prefetch_item.h",
     "prefetch_request_fetcher.cc",
     "prefetch_request_fetcher.h",
     "prefetch_service.h",
     "prefetch_service_impl.cc",
     "prefetch_service_impl.h",
+    "prefetch_types.h",
   ]
 
   deps = [
@@ -27,6 +30,7 @@ static_library("prefetch") {
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "prefetch_item_unittest.cc",
     "prefetch_request_fetcher_unittest.cc",
     "prefetch_service_impl_unittest.cc",
   ]