Deprecate NonThreadSafe in components/sync in favor of SequenceChecker.

components/sync/engine/attachments/fake_attachment_downloader.h

Index: components/sync/engine/attachments/fake_attachment_downloader.h
diff --git a/components/sync/engine/attachments/fake_attachment_downloader.h b/components/sync/engine/attachments/fake_attachment_downloader.h
index 6c572819f8f5558b485b0f672344b58f7628dc39..13eb7e2bf791573dd1a684d4d24987f8d3c5aac4 100644
--- a/components/sync/engine/attachments/fake_attachment_downloader.h
+++ b/components/sync/engine/attachments/fake_attachment_downloader.h
@@ -6,15 +6,14 @@
 #define COMPONENTS_SYNC_ENGINE_ATTACHMENTS_FAKE_ATTACHMENT_DOWNLOADER_H_
 
 #include "base/macros.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/sequence_checker.h"
 #include "components/sync/engine/attachments/attachment_downloader.h"
 
 namespace syncer {
 
 // FakeAttachmentDownloader is for tests. For every request it posts a success
 // callback with empty attachment.
-class FakeAttachmentDownloader : public AttachmentDownloader,
-                                 public base::NonThreadSafe {
+class FakeAttachmentDownloader : public AttachmentDownloader {
  public:
   FakeAttachmentDownloader();
   ~FakeAttachmentDownloader() override;
@@ -24,6 +23,8 @@ class FakeAttachmentDownloader : public AttachmentDownloader,
                           const DownloadCallback& callback) override;
 
  private:
+  SEQUENCE_CHECKER(sequence_checker_);
+
   DISALLOW_COPY_AND_ASSIGN(FakeAttachmentDownloader);
 };