Add glue for supporting custom MIME types in DataTransfer.

ui/base/clipboard/clipboard_mac.mm

Index: ui/base/clipboard/clipboard_mac.mm
diff --git a/ui/base/clipboard/clipboard_mac.mm b/ui/base/clipboard/clipboard_mac.mm
index e97cd8103c869f799ca7ff8b8f2489a60a9880df..ebeb90752c08b968382e056d537aecd955976ea1 100644
--- a/ui/base/clipboard/clipboard_mac.mm
+++ b/ui/base/clipboard/clipboard_mac.mm
@@ -284,6 +284,13 @@ SkBitmap Clipboard::ReadImage(Buffer buffer) const {
   return canvas.ExtractBitmap();
 }
 
+void Clipboard::ReadCustomData(Buffer buffer,
+                               const string16& type,
+                               string16* result) const {
+  // TODO(dcheng): Implement this.
+  NOTIMPLEMENTED();
+}
+
 void Clipboard::ReadBookmark(string16* title, std::string* url) const {
   NSPasteboard* pb = GetPasteboard();