Add stubs and glue for routing a request for an image from the clipboard.

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 89ad97d1f5b1833cb6bf4de6e41bfa735739d627..2761ebaf1a20cf65314fde855fbec40d595467bd 100644
--- a/ui/base/clipboard/clipboard_mac.mm
+++ b/ui/base/clipboard/clipboard_mac.mm
@@ -222,6 +222,14 @@ void Clipboard::ReadHTML(Clipboard::Buffer buffer, string16* markup,
     src_url->clear();
 }
 
+void Clipboard::ReadImage(Buffer buffer, std::string* data) const {
+  DCHECK_EQ(buffer, BUFFER_STANDARD);
+  if (!data) {
+    NOTREACHED();
+    return;
+  }
+}
+
 void Clipboard::ReadBookmark(string16* title, std::string* url) const {
   NSPasteboard* pb = GetPasteboard();