Implement ui::ClipboardMus.

ui/base/test/test_clipboard_unittest.cc

Index: ui/base/test/test_clipboard_unittest.cc
diff --git a/ui/base/test/test_clipboard_unittest.cc b/ui/base/test/test_clipboard_unittest.cc
index 8d3a4367428634669def78fcabeee8cd4d5ce4c7..da1b8fc357e5b3289e95189b0b63afb6d2166d17 100644
--- a/ui/base/test/test_clipboard_unittest.cc
+++ b/ui/base/test/test_clipboard_unittest.cc
@@ -6,11 +6,23 @@
 
 #include "testing/gtest/include/gtest/gtest.h"
 
+#if defined(USE_AURA)
+#include "ui/events/platform/platform_event_source.h"
+#endif
+
 namespace ui {
 
 struct TestClipboardTraits {
+#if defined(USE_AURA)
+  static std::unique_ptr<PlatformEventSource> GetEventSource() {
+    return nullptr;
+  }
+#endif
+
   static Clipboard* Create() { return TestClipboard::CreateForCurrentThread(); }
 
+  static bool IsMusTest() { return false; }
+
   static void Destroy(Clipboard* clipboard) {
     ASSERT_EQ(Clipboard::GetForCurrentThread(), clipboard);
     Clipboard::DestroyClipboardForCurrentThread();