Refactor MHTMLGenerator to allow getting sending the data to a file descriptor.

content/browser/download/save_package.cc

Index: content/browser/download/save_package.cc
diff --git a/content/browser/download/save_package.cc b/content/browser/download/save_package.cc
index 8a9781d82b07516e4a20ec100dee831895c75585..1b7d8a534d3250031d7ca562f2eea1b83654f2aa 100644
--- a/content/browser/download/save_package.cc
+++ b/content/browser/download/save_package.cc
@@ -363,7 +363,7 @@ void SavePackage::InitWithDownloadItem(
   }
 }
 
-void SavePackage::OnMHTMLGenerated(const base::FilePath& path, int64 size) {
+void SavePackage::OnMHTMLGenerated(int64 size) {
   if (size <= 0) {
     Cancel(false);
     return;