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..1be9ff14661882d07ec9a5a8c7379863216eb66b 100644
--- a/content/browser/download/download_stats.h
+++ b/content/browser/download/download_stats.h
@@ -145,6 +145,38 @@ enum DownloadDiscardReason {
   DOWNLOAD_DISCARD_DUE_TO_SHUTDOWN
 };
 
+// When parallel download is enabled, the download may fall back to a normal
+// download for various reasons. This enum counts the number of parallel
+// download and fallbacks. Also records the reasons why the download falls back
+// to a normal download. The reasons are not mutually exclusive.
+// Used in histogram "Download.ParallelDownload.CreationEvent".

[Ilya Sherman, 2017/04/10 21:54:15]

nit: Please also document that "and therefore, should be treated as
append-only".  Metrics data is persisted, so it's important that semantics
remain consistent over time.

[xingliu, 2017/04/10 22:07:12]

Done,

sorry I probably forgot this one.

+enum class ParallelDownloadCreationEvent {
+  // The total number of downloads started as parallel download.
+  STARTED_PARALLEL_DOWNLOAD = 0,
+
+  // The total number of downloads fell back to normal download when parallel
+  // download is enabled.
+  FELL_BACK_TO_NORMAL_DOWNLOAD,
+
+  // No ETag or Last-Modified response header.
+  FALLBACK_REASON_STRONG_VALIDATORS,
+
+  // No Accept-Range response header.
+  FALLBACK_REASON_ACCEPT_RANGE_HEADER,
+
+  // No Content-Length response header.
+  FALLBACK_REASON_CONTENT_LENGTH_HEADER,
+
+  // File size is not complied to finch configuration.
+  FALLBACK_REASON_FILE_SIZE,
+
+  // The HTTP connection type does not meet the requirement.
+  FALLBACK_REASON_CONNECTION_TYPE,
+
+  // Last entry of the enum.
+  COUNT,
+};
+
 // Increment one of the above counts.
 void RecordDownloadCount(DownloadCountTypes type);
 
@@ -236,6 +268,10 @@ void RecordParallelDownloadStats(
     size_t bytes_downloaded_without_parallel_streams,
     base::TimeDelta time_without_parallel_streams);
 
+// Records the parallel download creation counts and the reasons why the
+// download falls back to non-parallel download.
+void RecordParallelDownloadCreationEvent(ParallelDownloadCreationEvent event);
+
 // Record the result of a download file rename.
 void RecordDownloadFileRenameResultAfterRetry(
     base::TimeDelta time_since_first_failure,