Replace deprecated base::NonThreadSafe in chrome/browser/printing in favor of SequenceChecker.

chrome/browser/printing/background_printing_manager.h

Index: chrome/browser/printing/background_printing_manager.h
diff --git a/chrome/browser/printing/background_printing_manager.h b/chrome/browser/printing/background_printing_manager.h
index 6957e541066fb8d71cb93e29ae34ec76ffb23e89..ff0bd1e48378aedeac97a40eff62e0d84ea29335 100644
--- a/chrome/browser/printing/background_printing_manager.h
+++ b/chrome/browser/printing/background_printing_manager.h
@@ -11,7 +11,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/sequence_checker.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
@@ -26,8 +26,7 @@ namespace printing {
 // The hidden WebContents are no longer part of any Browser / TabStripModel.
 // The WebContents started life as a ConstrainedWebDialog.
 // They get deleted when the printing finishes.
-class BackgroundPrintingManager : public base::NonThreadSafe,
-                                  public content::NotificationObserver {
+class BackgroundPrintingManager : public content::NotificationObserver {
  public:
   class Observer;
 
@@ -65,6 +64,8 @@ class BackgroundPrintingManager : public base::NonThreadSafe,
 
   content::NotificationRegistrar registrar_;
 
+  SEQUENCE_CHECKER(sequence_checker_);
+
   DISALLOW_COPY_AND_ASSIGN(BackgroundPrintingManager);
 };