Revert of [Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/BUILD.gn

Index: sync/BUILD.gn
diff --git a/sync/BUILD.gn b/sync/BUILD.gn
index db83ae9d89f1016be597c2ffdd8052bee11679d3..4e830bcf21c379d6a5e2050f816de4c8c7b0166b 100644
--- a/sync/BUILD.gn
+++ b/sync/BUILD.gn
@@ -23,8 +23,6 @@
     "api/attachments/attachment_metadata.h",
     "api/attachments/attachment_store.cc",
     "api/attachments/attachment_store.h",
-    "api/attachments/attachment_store_backend.cc",
-    "api/attachments/attachment_store_backend.h",
     "api/string_ordinal.h",
     "api/sync_change.cc",
     "api/sync_change.h",
@@ -125,7 +123,7 @@
     "internal_api/attachments/attachment_service_impl.cc",
     "internal_api/attachments/attachment_service_proxy.cc",
     "internal_api/attachments/attachment_service_proxy_for_test.cc",
-    "internal_api/attachments/attachment_store_frontend.cc",
+    "internal_api/attachments/attachment_store_handle.cc",
     "internal_api/attachments/attachment_uploader.cc",
     "internal_api/attachments/attachment_uploader_impl.cc",
     "internal_api/attachments/attachment_util.cc",
@@ -165,7 +163,7 @@
     "internal_api/public/attachments/attachment_service_impl.h",
     "internal_api/public/attachments/attachment_service_proxy.h",
     "internal_api/public/attachments/attachment_service_proxy_for_test.h",
-    "internal_api/public/attachments/attachment_store_frontend.h",
+    "internal_api/public/attachments/attachment_store_handle.h",
     "internal_api/public/attachments/attachment_uploader.h",
     "internal_api/public/attachments/attachment_uploader_impl.h",
     "internal_api/public/attachments/attachment_util.h",
@@ -574,7 +572,7 @@
     "internal_api/attachments/attachment_downloader_impl_unittest.cc",
     "internal_api/attachments/attachment_service_impl_unittest.cc",
     "internal_api/attachments/attachment_service_proxy_unittest.cc",
-    "internal_api/attachments/attachment_store_frontend_unittest.cc",
+    "internal_api/attachments/attachment_store_handle_unittest.cc",
     "internal_api/attachments/attachment_store_test_template.h",
     "internal_api/attachments/attachment_uploader_impl_unittest.cc",
     "internal_api/attachments/fake_attachment_downloader_unittest.cc",