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 f46c0445affc4cbd324dca5e39328ec6d8dc5ff3..42bdb54f226bdb1b056957a6f9e5925c208190ac 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",
@@ -20,6 +22,8 @@ static_library("prefetch") {
     "prefetch_gcm_app_handler.cc",
     "prefetch_gcm_app_handler.h",
     "prefetch_gcm_handler.h",
+    "prefetch_in_memory_store.cc",
+    "prefetch_in_memory_store.h",
     "prefetch_item.cc",
     "prefetch_item.h",
     "prefetch_proto_utils.cc",
@@ -29,6 +33,7 @@ static_library("prefetch") {
     "prefetch_service.h",
     "prefetch_service_impl.cc",
     "prefetch_service_impl.h",
+    "prefetch_store.h",
     "prefetch_types.cc",
     "prefetch_types.h",
     "suggested_articles_observer.cc",