Revert of Implement ui::ClipboardMus.

base/threading/thread_restrictions.h

Index: base/threading/thread_restrictions.h
diff --git a/base/threading/thread_restrictions.h b/base/threading/thread_restrictions.h
index e916ecc6f3b954e08127e55c5d442b12895f0866..8d937b7468d80695863803317c72c164eca9110b 100644
--- a/base/threading/thread_restrictions.h
+++ b/base/threading/thread_restrictions.h
@@ -82,7 +82,6 @@
 }
 
 namespace views {
-class ClipboardMus;
 class ScreenMus;
 }
 
@@ -236,10 +235,6 @@
 #if !defined(OFFICIAL_BUILD)
   friend class content::SoftwareOutputDeviceMus;  // Interim non-production code
 #endif
-  // In the non-mus case, we called blocking OS functions in the ui::Clipboard
-  // implementation which weren't caught by threading restrictions. Our
-  // blocking calls to mus, however, are.
-  friend class views::ClipboardMus;
   friend class views::ScreenMus;
 // END USAGE THAT NEEDS TO BE FIXED.