Keep track of which attachments are referenced by which sync entries.

sync/api/attachments/attachment_service_proxy.cc

Index: sync/api/attachments/attachment_service_proxy.cc
diff --git a/sync/api/attachments/attachment_service_proxy.cc b/sync/api/attachments/attachment_service_proxy.cc
index 9bc40e5530ca745a403c7e71e0a3032e9792215d..6590033df531b2855ccdd0b85d6ac74b38122519 100644
--- a/sync/api/attachments/attachment_service_proxy.cc
+++ b/sync/api/attachments/attachment_service_proxy.cc
@@ -35,6 +35,15 @@ void ProxyDropCallback(
   task_runner->PostTask(FROM_HERE, base::Bind(callback, result));
 }
 
+// Invokes |callback| with |result| and |attachments| in the |task_runner|
+// thread.
+void ProxyStoreCallback(
+    const scoped_refptr<base::SequencedTaskRunner>& task_runner,
+    const AttachmentService::StoreCallback& callback,
+    const AttachmentService::StoreResult& result) {
+  task_runner->PostTask(FROM_HERE, base::Bind(callback, result));
+}
+
 }  // namespace
 
 AttachmentServiceProxy::AttachmentServiceProxy() {
@@ -85,11 +94,17 @@ void AttachmentServiceProxy::DropAttachments(
                                             proxy_callback));
 }
 
-void AttachmentServiceProxy::OnSyncDataAdd(const SyncData& sync_data) {
+void AttachmentServiceProxy::StoreAttachments(const AttachmentList& attachments,
+                                              const StoreCallback& callback) {
   DCHECK(wrapped_task_runner_);
+  StoreCallback proxy_callback = base::Bind(
+      &ProxyStoreCallback, base::MessageLoopProxy::current(), callback);
   wrapped_task_runner_->PostTask(
       FROM_HERE,
-      base::Bind(&AttachmentService::OnSyncDataAdd, core_, sync_data));
+      base::Bind(&AttachmentService::StoreAttachments,
+                 core_,
+                 attachments,
+                 proxy_callback));
 }
 
 void AttachmentServiceProxy::OnSyncDataDelete(const SyncData& sync_data) {
@@ -137,11 +152,13 @@ void AttachmentServiceProxy::Core::DropAttachments(
   wrapped_->DropAttachments(attachment_ids, callback);
 }
 
-void AttachmentServiceProxy::Core::OnSyncDataAdd(const SyncData& sync_data) {
+void AttachmentServiceProxy::Core::StoreAttachments(
+    const AttachmentList& attachments,
+    const StoreCallback& callback) {
   if (!wrapped_) {
     return;
   }
-  wrapped_->OnSyncDataAdd(sync_data);
+  wrapped_->StoreAttachments(attachments, callback);
 }
 
 void AttachmentServiceProxy::Core::OnSyncDataDelete(const SyncData& sync_data) {