Adds tracing to MHTML generation call chain.

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 50d5bebc702bc782b3535cd0b19508558f67f240..f3a838c9682ab989a8a41b9dfc4012af84342c51 100644
--- a/content/browser/download/mhtml_generation_manager.h
+++ b/content/browser/download/mhtml_generation_manager.h
@@ -77,9 +77,9 @@ class MHTMLGenerationManager {
   void OnFileClosed(int job_id, JobStatus job_status, int64_t file_size);
 
   // Creates and registers a new job.
-  int NewJob(WebContents* web_contents,
-             const MHTMLGenerationParams& params,
-             const GenerateMHTMLCallback& callback);
+  Job* NewJob(WebContents* web_contents,
+              const MHTMLGenerationParams& params,
+              const GenerateMHTMLCallback& callback);
 
   // Finds job by id.  Returns nullptr if no job with a given id was found.
   Job* FindJob(int job_id);