Implementation of OnDiskAttachmentStore.

sync/BUILD.gn

Index: sync/BUILD.gn
diff --git a/sync/BUILD.gn b/sync/BUILD.gn
index 2ba15cb0805d782405e13e0b66b3e8483e5334ff..fc97468a290682e8f4d78f2b9e88c8416bcb7c7d 100644
--- a/sync/BUILD.gn
+++ b/sync/BUILD.gn
@@ -126,6 +126,7 @@ source_set("sync_core") {
     "internal_api/attachments/fake_attachment_downloader.cc",
     "internal_api/attachments/fake_attachment_uploader.cc",
     "internal_api/attachments/in_memory_attachment_store.cc",
+    "internal_api/attachments/on_disk_attachment_store.cc",
     "internal_api/base_node.cc",
     "internal_api/base_transaction.cc",
     "internal_api/change_record.cc",
@@ -164,6 +165,7 @@ source_set("sync_core") {
     "internal_api/public/attachments/fake_attachment_downloader.h",
     "internal_api/public/attachments/fake_attachment_uploader.h",
     "internal_api/public/attachments/in_memory_attachment_store.h",
+    "internal_api/public/attachments/on_disk_attachment_store.h",
     "internal_api/public/base/attachment_id_proto.cc",
     "internal_api/public/base/attachment_id_proto.h",
     "internal_api/public/base/cancelation_observer.cc",
@@ -565,9 +567,12 @@ test("sync_unit_tests") {
     "internal_api/attachments/attachment_service_impl_unittest.cc",
     "internal_api/attachments/attachment_service_proxy_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",
     "internal_api/attachments/fake_attachment_uploader_unittest.cc",
+    "internal_api/attachments/in_memory_attachment_store_unittest.cc",
+    "internal_api/attachments/on_disk_attachment_store_unittest.cc",
     "internal_api/debug_info_event_listener_unittest.cc",
     "internal_api/http_bridge_unittest.cc",
     "internal_api/js_mutation_event_observer_unittest.cc",