Fix a document memory leak in drag-and-drop.

Source/core/clipboard/Clipboard.cpp

Index: Source/core/clipboard/Clipboard.cpp
diff --git a/Source/core/clipboard/Clipboard.cpp b/Source/core/clipboard/Clipboard.cpp
index a05a87fa062678402b3cee6e91eb7a2a9dfa155d..c80250cd23e82bebc153018b8dfca69fec319ca8 100644
--- a/Source/core/clipboard/Clipboard.cpp
+++ b/Source/core/clipboard/Clipboard.cpp
@@ -228,6 +228,16 @@ void Clipboard::setDragImage(Element* image, int x, int y, ExceptionState& excep
         setDragImageElement(image, location);
 }
 
+void Clipboard::clearDragImage()
+{
+    if (!canSetDragImage())
+        return;
+
+    m_dragImage = 0;
+    m_dragLoc = IntPoint();
+    m_dragImageElement = nullptr;
+}
+
 void Clipboard::setDragImageResource(ImageResource* img, const IntPoint& loc)
 {
     setDragImage(img, 0, loc);