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

content/browser/download/download_stats.h

Index: content/browser/download/download_stats.h
diff --git a/content/browser/download/download_stats.h b/content/browser/download/download_stats.h
index 0c4a5be726edf4de5dd19f07118aec46ae9dce35..d6d8c9d9a9a713b604185378625d2107b66f9dff 100644
--- a/content/browser/download/download_stats.h
+++ b/content/browser/download/download_stats.h
@@ -145,6 +145,18 @@ enum DownloadDiscardReason {
   DOWNLOAD_DISCARD_DUE_TO_SHUTDOWN
 };
 
+// The reason why parallel download is not applicable and fallbacks to a
+// non-parallel download.
+enum ParallelDownloadFailReason {

[Ilya Sherman, 2017/04/07 22:13:08]

nit: Could this be an enum class?

[xingliu, 2017/04/08 01:13:28]

Done.

+  STRONG_VALIDATORS = 0,
+  ACCEPT_RANGE_HEADER,
+  CONTENT_LENGTH_HEADER,
+  FILE_SIZE,
+  CONNECTION_TYPE,
+  FAIL_COUNT,

[Ilya Sherman, 2017/04/07 22:13:08]

What does "FAIL_COUNT" mean?  "EXCEEDED_MAX_ALLOWED_FAILURES"?

[xingliu, 2017/04/08 01:13:28]

Changed to FALLBACK_TO_NORMAL_DOWNLOAD_COUNT.

+  MAX_COUNT,

[Ilya Sherman, 2017/04/07 22:13:08]

nit: I find "MAX_COUNT" to be a confusing name.  Could this just be "COUNT"? 
(Assuming the name above doesn't need "count" in it.)

[xingliu, 2017/04/08 01:13:28]

Done. Reworked the naming and the comments.

+};
+
 // Increment one of the above counts.
 void RecordDownloadCount(DownloadCountTypes type);
 
@@ -236,6 +248,9 @@ void RecordParallelDownloadStats(
     size_t bytes_downloaded_without_parallel_streams,
     base::TimeDelta time_without_parallel_streams);
 
+// Records the reason why the download is not created as parallel download.
+void RecordParallelDownloadFailReason(ParallelDownloadFailReason reason);

[qinmin, 2017/04/07 21:00:52]

This sounds more like a download failed. what about
RecordParallelRequestCreationFailureReason or sth like that

[xingliu, 2017/04/08 01:13:27]

Done.

+
 // Record the result of a download file rename.
 void RecordDownloadFileRenameResultAfterRetry(
     base::TimeDelta time_since_first_failure,