Implemented ExternalData interface on DownloadItem and used it for SafeBrowsing.

content/browser/download/download_item.h

Index: content/browser/download/download_item.h
diff --git a/content/browser/download/download_item.h b/content/browser/download/download_item.h
index c51f35e22e2499d236f6f980219704aa881a18c9..ccc85730036cdde48c78baf1edc4500f12164904 100644
--- a/content/browser/download/download_item.h
+++ b/content/browser/download/download_item.h
@@ -18,6 +18,7 @@
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_ITEM_H_
 #pragma once
 
+#include <map>
 #include <string>
 
 #include "base/string16.h"
@@ -100,6 +101,13 @@ class CONTENT_EXPORT DownloadItem {
     virtual ~Observer() {}
   };
 
+  // Interface for data that can be stored associated with (and owned
+  // by) an object of this class via GetExternalData/SetExternalData.
+  class ExternalData {
+   public:
+    virtual ~ExternalData() {};
+  };
+
   virtual ~DownloadItem();
 
   virtual void AddObserver(DownloadItem::Observer* observer) = 0;
@@ -308,6 +316,20 @@ class CONTENT_EXPORT DownloadItem {
   // rewrites of the DownloadManager queues.
   virtual void OffThreadCancel(DownloadFileManager* file_manager) = 0;
 
+  // Manage data owned by other subsystems associated with the
+  // DownloadItem.  By custom, key is the address of a
+  // static char subsystem_specific_string[] = ".."; defined
+  // in the subsystem, but the only requirement of this interface
+  // is that the key be unique over all data stored with this
+  // DownloadItem.
+  //
+  // Note that SetExternalData takes ownership of the
+  // passed object; it will be destroyed when the DownloadItem is.
+  // If an object is already held by the DownloadItem associated with
+  // the passed key, it will be destroyed on overwrite.

[asanka, 2011/12/07 04:10:57]

Nit: Should this mention the behavior when the object already held is the same
as the new one being set?

[Randy Smith (Not in Mondays), 2011/12/07 18:36:12]

Done.

+  virtual ExternalData* GetExternalData(const void* key) = 0;
+  virtual void SetExternalData(const void* key, ExternalData* data) = 0;
+
   virtual std::string DebugString(bool verbose) const = 0;
 
   virtual void MockDownloadOpenForTesting() = 0;