Replace the DownloadFileManager with direct ownership of DownloadFileImpl

content/browser/download/base_file.h

Index: content/browser/download/base_file.h
diff --git a/content/browser/download/base_file.h b/content/browser/download/base_file.h
index 885c79df738aa624318c59ecf14be289f245d2f3..f7f9c62402393804dda769b87108fa9973526ef5 100644
--- a/content/browser/download/base_file.h
+++ b/content/browser/download/base_file.h
@@ -29,6 +29,8 @@ class FileStream;
 // for DownloadFile and SaveFile, which keep more state information.
 class CONTENT_EXPORT BaseFile {
  public:
+  // May be constructed on any thread.  All other routines (including
+  // destruction) must occur on the FILE thread.
   BaseFile(const FilePath& full_path,
            const GURL& source_url,
            const GURL& referrer_url,