Make DownloadItem derive SupportsUserData instead of re-implementing it.

chrome/browser/download/download_completion_blocker.h

Index: chrome/browser/download/download_completion_blocker.h
diff --git a/chrome/browser/download/download_completion_blocker.h b/chrome/browser/download/download_completion_blocker.h
index 8f369b5a01fd8fbccb6320306505b7938caef159..6cce983e38cf5c7f7f9fb7599bab71abb4bf230c 100644
--- a/chrome/browser/download/download_completion_blocker.h
+++ b/chrome/browser/download/download_completion_blocker.h
@@ -6,13 +6,14 @@
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_COMPLETION_BLOCKER_H_
 
 #include "base/callback.h"
+#include "base/supports_user_data.h"
 #include "content/public/browser/download_item.h"
 
 // A subsystem may use a DownloadCompletionBlocker in conjunction with
 // DownloadManagerDelegate::ShouldCompleteDownload() in order to block the
 // completion of a DownloadItem. CompleteDownload() will run the most recent
 // callback set.
-class DownloadCompletionBlocker : public content::DownloadItem::ExternalData {
+class DownloadCompletionBlocker : public base::SupportsUserData::Data {
  public:
   DownloadCompletionBlocker();
   virtual ~DownloadCompletionBlocker();