Change all platforms except Windows to access the clipboard solely from the UI thread

ui/base/clipboard/clipboard.h

Index: ui/base/clipboard/clipboard.h
diff --git a/ui/base/clipboard/clipboard.h b/ui/base/clipboard/clipboard.h
index 98904c0065f971759808f5cfe82162d082b11d24..a83c59b520725789aa6ec86412e0ae2352b83086 100644
--- a/ui/base/clipboard/clipboard.h
+++ b/ui/base/clipboard/clipboard.h
@@ -10,10 +10,12 @@
 #include <string>
 #include <vector>
 
+#include "base/compiler_specific.h"
 #include "base/gtest_prod_util.h"
 #include "base/process.h"
 #include "base/shared_memory.h"
 #include "base/string16.h"
+#include "base/threading/thread_checker.h"
 #include "ui/base/ui_export.h"
 
 #if defined(TOOLKIT_USES_GTK)
@@ -46,7 +48,7 @@ class NSString;
 
 namespace ui {
 
-class UI_EXPORT Clipboard {
+class UI_EXPORT Clipboard : NON_EXPORTED_BASE(public base::ThreadChecker) {
  public:
   // MIME type constants.
   static const char kMimeTypeText[];