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

sync/internal_api/attachments/attachment_service_proxy_unittest.cc

Index: sync/internal_api/attachments/attachment_service_proxy_unittest.cc
diff --git a/sync/internal_api/attachments/attachment_service_proxy_unittest.cc b/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
index 41af1b233710914b1e42caf3cd25a96c9f6727b2..a66b39f4858fc278c327bab10a694319a529e5d2 100644
--- a/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
+++ b/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
@@ -33,8 +33,6 @@ class StubAttachmentService : public AttachmentService,
 
   ~StubAttachmentService() override {}
 
-  AttachmentStore* GetStore() override { return NULL; }
-
   void GetOrDownloadAttachments(
       const AttachmentIdList& attachment_ids,
       const GetOrDownloadCallback& callback) override {
@@ -132,10 +130,6 @@ class AttachmentServiceProxyTest : public testing::Test,
   int count_callback_get_or_download;
 };
 
-TEST_F(AttachmentServiceProxyTest, GetStore) {
-  EXPECT_EQ(NULL, proxy->GetStore());
-}
-
 // Verify that each of AttachmentServiceProxy's methods are invoked on the stub.
 // Verify that the methods that take callbacks invoke passed callbacks on this
 // thread.