Add glue for supporting custom MIME types in DataTransfer.

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 9c010e4fbae812a251479308499d99f81861cd70..2cac34805f3ceba80469bc96d6f9a5800c725ac0 100644
--- a/ui/base/clipboard/clipboard.h
+++ b/ui/base/clipboard/clipboard.h
@@ -156,6 +156,9 @@ class UI_EXPORT Clipboard {
   // Reads an image from the clipboard, if available.
   SkBitmap ReadImage(Buffer buffer) const;
 
+  void ReadCustomData(Buffer buffer, const string16& type,
+                      string16* data) const;
+
   // Reads a bookmark from the clipboard, if available.
   void ReadBookmark(string16* title, std::string* url) const;