Add FakeClipboard implementation for unit tests.

ui/base/clipboard/clipboard_mac.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_BASE_CLIPBOARD_CLIPBOARD_MAC_H_
 #define UI_BASE_CLIPBOARD_CLIPBOARD_MAC_H_
 
 #include "ui/base/clipboard/clipboard.h"
 
 namespace ui {
 
 class ClipboardMac : public Clipboard {
  private:
   friend class Clipboard;
 
   ClipboardMac();
   ~ClipboardMac() override;
 
   // Clipboard overrides:
-  uint64 GetSequenceNumber(ClipboardType type) override;
+  uint64 GetSequenceNumber(ClipboardType type) const override;
   bool IsFormatAvailable(const FormatType& format,
                          ClipboardType type) const override;
   void Clear(ClipboardType type) override;
   void ReadAvailableTypes(ClipboardType type,
                           std::vector<base::string16>* types,
                           bool* contains_filenames) const override;
   void ReadText(ClipboardType type, base::string16* result) const override;
   void ReadAsciiText(ClipboardType type, std::string* result) const override;
   void ReadHTML(ClipboardType type,
                 base::string16* markup,
@@ skipping 23 matching lines @@
   void WriteData(const FormatType& format,
                  const char* data_data,
                  size_t data_len) override;
 
   DISALLOW_COPY_AND_ASSIGN(ClipboardMac);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_CLIPBOARD_CLIPBOARD_MAC_H_