Add UMA to track why a download is non-parallel when enabled parallel

content/browser/download/download_stats.cc

Index: content/browser/download/download_stats.cc
diff --git a/content/browser/download/download_stats.cc b/content/browser/download/download_stats.cc
index 92533fbf3d2d0efbe24ead9b677bc553dcb9d9f3..72135e29961bb7eec5104228af86ccddaef3d062 100644
--- a/content/browser/download/download_stats.cc
+++ b/content/browser/download/download_stats.cc
@@ -816,6 +816,11 @@ void RecordParallelDownloadStats(
   }
 }
 
+void RecordParallelDownloadCreationEvent(ParallelDownloadCreationEvent event) {
+  UMA_HISTOGRAM_ENUMERATION("Download.ParallelDownload.CreationEvent", event,
+                            ParallelDownloadCreationEvent::COUNT);
+}
+
 void RecordDownloadFileRenameResultAfterRetry(
     base::TimeDelta time_since_first_failure,
     DownloadInterruptReason interrupt_reason) {