Save As for content retrieved via POST works in most circumstances.

content/test/mock_download_manager.h

Index: content/test/mock_download_manager.h
diff --git a/content/test/mock_download_manager.h b/content/test/mock_download_manager.h
index 280bc8aaa91120dc8a2739865cc27d9265fb4b34..be946b57eb65a8ae0497b24fdac5b8d58c86e602 100644
--- a/content/test/mock_download_manager.h
+++ b/content/test/mock_download_manager.h
@@ -51,10 +51,11 @@ class MockDownloadManager : public content::DownloadManager {
                                            base::Time remove_end));
   MOCK_METHOD1(RemoveDownloads, int(base::Time remove_begin));
   MOCK_METHOD0(RemoveAllDownloads, int());
-  MOCK_METHOD6(DownloadUrl, void(const GURL& url,
+  MOCK_METHOD7(DownloadUrl, void(const GURL& url,
                                  const GURL& referrer,
                                  const std::string& referrer_encoding,
                                  bool prefer_cache,
+                                 int64 post_id,
                                  const DownloadSaveInfo& save_info,
                                  content::WebContents* web_contents));
   MOCK_METHOD1(AddObserver, void(Observer* observer));