Add FakeClipboard implementation for unit tests.

ui/base/clipboard/clipboard_mac.mm

 // Copyright (c) 2012 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.
 
 #include "ui/base/clipboard/clipboard_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
@@ skipping 166 matching lines @@
 
 // ClipboardMac implementation.
 ClipboardMac::ClipboardMac() {
   DCHECK(CalledOnValidThread());
 }
 
 ClipboardMac::~ClipboardMac() {
   DCHECK(CalledOnValidThread());
 }
 
-uint64 ClipboardMac::GetSequenceNumber(ClipboardType type) {
+uint64 ClipboardMac::GetSequenceNumber(ClipboardType type) const {
   DCHECK(CalledOnValidThread());
   DCHECK_EQ(type, CLIPBOARD_TYPE_COPY_PASTE);
 
   NSPasteboard* pb = GetPasteboard();
   return [pb changeCount];
 }
 
 bool ClipboardMac::IsFormatAvailable(const FormatType& format,
                                      ClipboardType type) const {
   DCHECK(CalledOnValidThread());
@@ skipping 250 matching lines @@
 // Write an extra flavor that signifies WebKit was the last to modify the
 // pasteboard. This flavor has no data.
 void ClipboardMac::WriteWebSmartPaste() {
   NSPasteboard* pb = GetPasteboard();
   NSString* format = GetWebKitSmartPasteFormatType().ToNSString();
   [pb addTypes:[NSArray arrayWithObject:format] owner:nil];
   [pb setData:nil forType:format];
 }
 
 }  // namespace ui