Implemented ExternalData interface on DownloadItem and used it for SafeBrowsing.

content/browser/download/download_item_impl.h

Index: content/browser/download/download_item_impl.h
diff --git a/content/browser/download/download_item_impl.h b/content/browser/download/download_item_impl.h
index 5c3f3b97dc414798f5e6d4f5ec7ceb49f9da868d..52699ce5d94b79793d1a78368b2e17177e970f0d 100644
--- a/content/browser/download/download_item_impl.h
+++ b/content/browser/download/download_item_impl.h
@@ -202,6 +202,8 @@ class CONTENT_EXPORT DownloadItemImpl : public DownloadItem {
   virtual void OffThreadCancel(DownloadFileManager* file_manager) OVERRIDE;
   virtual std::string DebugString(bool verbose) const OVERRIDE;
   virtual void MockDownloadOpenForTesting() OVERRIDE;
+  virtual ExternalData* GetExternalData(const void* key) OVERRIDE;
+  virtual void SetExternalData(const void* key, ExternalData* data) OVERRIDE;
 
  private:
   // Construction common to all constructors. |active| should be true for new
@@ -352,6 +354,10 @@ class CONTENT_EXPORT DownloadItemImpl : public DownloadItem {
   // Did the delegate delay calling Complete on this download?
   bool delegate_delayed_complete_;
 
+  // External Data storage.  All objects in the store
+  // are owned by the DownloadItemImpl.
+  std::map<const void*, ExternalData*> external_data_map_;
+
   DISALLOW_COPY_AND_ASSIGN(DownloadItemImpl);
 };