mac: Don't lose pixels when copying a retina image from the clipboard.

ui/base/clipboard/clipboard_mac.h

Index: ui/base/clipboard/clipboard_mac.h
diff --git a/ui/base/clipboard/clipboard_mac.h b/ui/base/clipboard/clipboard_mac.h
index 3497adc2d46a8da0ddedbe02c726e270aebd2448..2e264cb0171966e4891200c58d4b276dd01df703 100644
--- a/ui/base/clipboard/clipboard_mac.h
+++ b/ui/base/clipboard/clipboard_mac.h
@@ -11,12 +11,13 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "ui/base/clipboard/clipboard.h"
+#include "ui/base/ui_base_export.h"
 
 @class NSPasteboard;
 
 namespace ui {
 
-class ClipboardMac : public Clipboard {
+class UI_BASE_EXPORT ClipboardMac : public Clipboard {
  private:
   FRIEND_TEST_ALL_PREFIXES(ClipboardMacTest, ReadImageRetina);
   FRIEND_TEST_ALL_PREFIXES(ClipboardMacTest, ReadImageNonRetina);