[Downloads] Use the initiating StoragePartition for resumption.

content/public/browser/download_item.h

Index: content/public/browser/download_item.h
diff --git a/content/public/browser/download_item.h b/content/public/browser/download_item.h
index 6182cd4185b6e38e4bb511dd3fe834d3f5fecd30..38164bf6687e918742e90ec60906278e5ad5f62f 100644
--- a/content/public/browser/download_item.h
+++ b/content/public/browser/download_item.h
@@ -192,6 +192,7 @@ class CONTENT_EXPORT DownloadItem : public base::SupportsUserData {
   virtual const std::vector<GURL>& GetUrlChain() const = 0;
   virtual const GURL& GetOriginalUrl() const = 0;
   virtual const GURL& GetReferrerUrl() const = 0;
+  virtual const GURL& GetSiteInstanceUrl() const = 0;

[Charlie Reis, 2016/05/05 21:43:22]

This should really be GetSiteURL.  SiteInstance is one instance of a
Site, but the Site URL is the same for all instances of it.  (Applies throughout
the CL.)

I see that there aren't any comments in this section, but it seems like we might
want to add a comment on this saying it's used to determine StoragePartition. 
(There are many uses of SiteInstance::GetSiteURL that are currently incorrect
that nick@ is cleaning up, but this one should be safe.)

[ncarter (slow), 2016/05/05 23:29:07]

Agree that this is safe.

[asanka, 2016/05/07 00:24:45]

Cool. Thanks. I added a comment here and changed the names to
site_url/GetSiteUrl().

   virtual const GURL& GetTabUrl() const = 0;
   virtual const GURL& GetTabReferrerUrl() const = 0;
   virtual std::string GetSuggestedFilename() const = 0;