Deprecate NonThreadSafe in components/sync in favor of SequenceChecker.

components/sync/engine/attachments/attachment_store_frontend.h

Index: components/sync/engine/attachments/attachment_store_frontend.h
diff --git a/components/sync/engine/attachments/attachment_store_frontend.h b/components/sync/engine/attachments/attachment_store_frontend.h
index fc1b14eb22abc029b25a87ba4a5c10e7162d1402..c1468960e8b60934b2d5b34f5dfa5538dae18194 100644
--- a/components/sync/engine/attachments/attachment_store_frontend.h
+++ b/components/sync/engine/attachments/attachment_store_frontend.h
@@ -9,7 +9,7 @@
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/sequence_checker.h"
 #include "components/sync/model/attachments/attachment_store.h"
 
 namespace base {
@@ -30,8 +30,7 @@ class AttachmentStoreBackend;
 // Method signatures of AttachmentStoreFrontend match exactly methods of
 // AttachmentStoreBackend.
 class AttachmentStoreFrontend
-    : public base::RefCounted<AttachmentStoreFrontend>,
-      public base::NonThreadSafe {
+    : public base::RefCounted<AttachmentStoreFrontend> {
  public:
   AttachmentStoreFrontend(
       std::unique_ptr<AttachmentStoreBackend> backend,
@@ -66,6 +65,8 @@ class AttachmentStoreFrontend
   std::unique_ptr<AttachmentStoreBackend> backend_;
   scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
 
+  SEQUENCE_CHECKER(sequence_checker_);
+
   DISALLOW_COPY_AND_ASSIGN(AttachmentStoreFrontend);
 };