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 4dc7a380557640a94b1d6cf426d6712104a1987a..ad82d0611b9878093510c62893f8aca38cd976ed 100644
--- a/ui/base/clipboard/clipboard.h
+++ b/ui/base/clipboard/clipboard.h
@@ -14,6 +14,7 @@
 #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)
@@ -39,7 +40,7 @@ class NSString;
 
 namespace ui {
 
-class UI_EXPORT Clipboard {
+class UI_EXPORT Clipboard : public base::ThreadChecker {
  public:
   // Platform neutral holder for native data representation of a clipboard type.
   struct UI_EXPORT FormatType {
@@ -208,7 +209,6 @@ class UI_EXPORT Clipboard {
   // Reads a file or group of files from the clipboard, if available, into the
   // out parameter.
   void ReadFile(FilePath* file) const;
-  void ReadFiles(std::vector<FilePath>* files) const;
 
   // Reads raw data from the clipboard with the given format type. Stores result
   // as a byte vector.
@@ -253,6 +253,8 @@ class UI_EXPORT Clipboard {
   FRIEND_TEST_ALL_PREFIXES(ClipboardTest, SharedBitmapTest);
   FRIEND_TEST_ALL_PREFIXES(ClipboardTest, EmptyHTMLTest);
 
+  void ReadFiles(std::vector<FilePath>* files) const;
+
   void DispatchObject(ObjectType type, const ObjectMapParams& params);
 
   void WriteText(const char* text_data, size_t text_len);