Add UMA histograms to MHTML save operations.

content/browser/download/mhtml_generation_manager.h

Index: content/browser/download/mhtml_generation_manager.h
diff --git a/content/browser/download/mhtml_generation_manager.h b/content/browser/download/mhtml_generation_manager.h
index f3a838c9682ab989a8a41b9dfc4012af84342c51..31d9a64df624431324431721353b62e1eb2956a1 100644
--- a/content/browser/download/mhtml_generation_manager.h
+++ b/content/browser/download/mhtml_generation_manager.h
@@ -53,7 +53,8 @@ class MHTMLGenerationManager {
       RenderFrameHostImpl* sender,
       int job_id,
       bool mhtml_generation_in_renderer_succeeded,
-      const std::set<std::string>& digests_of_uris_of_serialized_resources);
+      const std::set<std::string>& digests_of_uris_of_serialized_resources,
+      base::TimeDelta renderer_main_thread_time);
 
  private:
   friend struct base::DefaultSingletonTraits<MHTMLGenerationManager>;