Clean up ui::ClipboardMonitor.

- Use a ThreadChecker member instead of inheriting from ThreadChecker.
- Switch from a leaky Singleton to base::NoDestructor.

Change-Id: I86f8f50f7776ceefa62ab3586314c5af3bacf7e7
Reviewed-on: https://chromium-review.googlesource.com/1178468Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583866}

ui/base/clipboard/clipboard_monitor.cc

@@ -12,28 +12,28 @@ namespace ui {
 ClipboardMonitor::ClipboardMonitor() {}
 
 ClipboardMonitor::~ClipboardMonitor() {
-  DCHECK(CalledOnValidThread());
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
 }
 
 // static
 ClipboardMonitor* ClipboardMonitor::GetInstance() {
-  return base::Singleton<ClipboardMonitor,
-                         base::LeakySingletonTraits<ClipboardMonitor>>::get();
+  static base::NoDestructor<ClipboardMonitor> monitor;
+  return monitor.get();
 }
 
 void ClipboardMonitor::NotifyClipboardDataChanged() {
-  DCHECK(CalledOnValidThread());
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   for (ClipboardObserver& observer : observers_)
     observer.OnClipboardDataChanged();
 }
 
 void ClipboardMonitor::AddObserver(ClipboardObserver* observer) {
-  DCHECK(CalledOnValidThread());
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   observers_.AddObserver(observer);
 }
 
 void ClipboardMonitor::RemoveObserver(ClipboardObserver* observer) {
-  DCHECK(CalledOnValidThread());
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   observers_.RemoveObserver(observer);
 }
 

ui/base/clipboard/clipboard_monitor.h

@@ -6,7 +6,7 @@
 #define UI_BASE_CLIPBOARD_CLIPBOARD_MONITOR_H_
 
 #include "base/macros.h"
-#include "base/memory/singleton.h"
+#include "base/no_destructor.h"
 #include "base/observer_list.h"
 #include "base/threading/thread_checker.h"
 #include "ui/base/ui_base_export.h"
@@ -17,7 +17,7 @@ class ClipboardObserver;
 
 // A singleton instance to monitor and notify ClipboardObservers for clipboard
 // changes.
-class UI_BASE_EXPORT ClipboardMonitor : public base::ThreadChecker {
+class UI_BASE_EXPORT ClipboardMonitor {
  public:
   static ClipboardMonitor* GetInstance();
 
@@ -31,15 +31,18 @@ class UI_BASE_EXPORT ClipboardMonitor : public base::ThreadChecker {
   virtual void NotifyClipboardDataChanged();
 
  private:
-  friend struct base::DefaultSingletonTraits<ClipboardMonitor>;
+  friend class base::NoDestructor<ClipboardMonitor>;
+
   ClipboardMonitor();
   virtual ~ClipboardMonitor();
 
   base::ObserverList<ClipboardObserver> observers_;
 
+  THREAD_CHECKER(thread_checker_);
+
   DISALLOW_COPY_AND_ASSIGN(ClipboardMonitor);
 };
 
 }  // namespace ui
 
-#endif /* UI_BASE_CLIPBOARD_CLIPBOARD_MONITOR_H_ */
+#endif  // UI_BASE_CLIPBOARD_CLIPBOARD_MONITOR_H_