Add UMA metric to track parallel download requests stats.

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 3fcff94ed6e5de4685d2882b858da4f8f7c4e674..eeac14a4ae0553b430e115033d923cf107d1b9d4 100644
--- a/content/browser/download/download_stats.cc
+++ b/content/browser/download/download_stats.cc
@@ -773,6 +773,20 @@ void RecordParallelDownloadCount(DownloadCountTypes type) {
                             DOWNLOAD_COUNT_TYPES_LAST_ENTRY);
 }
 
+void RecordParallelDownloadRequestCount(int request_count) {
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Download.ParallelDownloadRequestCount",
+                              request_count, 1, 10, 10);

[Ilya Sherman, 2017/03/31 00:33:04]

nit: Did you check that this gives you the desired bucket layout?  I suspect you
might want the last argument to be 11, to make space for the underflow bucket. 
In any case, I'd recommend double-checking locally via chrome://histograms that
this gives you the desired layout.

[xingliu, 2017/03/31 21:48:26]

Done, thanks.

+}
+
+void RecordParallelDownloadEarlyRequests(int request_count) {
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Download.ParallelDownloadEarlyRequestCount",
+                              request_count, 1, 10, 10);
+}
+
+void RecordParallelDownloadAddStreamSuccess(bool success) {
+  UMA_HISTOGRAM_BOOLEAN("Download.ParallelDownloadAddStreamSuccess", success);
+}
+
 void RecordParallelDownloadStats(
     size_t bytes_downloaded_with_parallel_streams,
     base::TimeDelta time_with_parallel_streams,