Fix the crash that could occur when the window is closed while web contents drag is in progress.

content/browser/web_contents/web_contents_drag_win.h

Index: content/browser/web_contents/web_contents_drag_win.h
diff --git a/content/browser/web_contents/web_contents_drag_win.h b/content/browser/web_contents/web_contents_drag_win.h
index f606d0fbe3c8edd5f5a1910ba8f22093f57599ae..f60401c51f5bdf3074629acebea8e6c1e7f42481 100644
--- a/content/browser/web_contents/web_contents_drag_win.h
+++ b/content/browser/web_contents/web_contents_drag_win.h
@@ -67,7 +67,11 @@ class CONTENT_EXPORT WebContentsDragWin
                                   ui::OSExchangeData* data);
   void PrepareDragForUrl(const WebDropData& drop_data,
                          ui::OSExchangeData* data);
-  void DoDragging(const WebDropData& drop_data,
+  // Returns false if the source window becomes invalid when the drag ends.
+  // This could happen when the window gets destroyed when the drag is still in
+  // progress. No further processing should be done beyond this return point
+  // because the instance has been destroyed.
+  bool DoDragging(const WebDropData& drop_data,
                   WebKit::WebDragOperationsMask ops,
                   const GURL& page_url,
                   const std::string& page_encoding,