Use ByteStream in downloads system to decouple source and sink.

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 40ed9123d8ebf4fcf09a9da2b998fbd8346158bc..756f9b7aafb04a2bf10297cda6e8dca25f226b2c 100644
--- a/content/browser/download/download_stats.cc
+++ b/content/browser/download/download_stats.cc
@@ -255,8 +255,7 @@ void RecordDownloadMimeType(const std::string& mime_type_string) {
 void RecordFileThreadReceiveBuffers(size_t num_buffers) {
     UMA_HISTOGRAM_CUSTOM_COUNTS(
       "Download.FileThreadReceiveBuffers", num_buffers, 1,
-      DownloadResourceHandler::kLoadsToWrite,
-      DownloadResourceHandler::kLoadsToWrite);
+      100, 100);
 }
 
 void RecordBandwidth(double actual_bandwidth, double potential_bandwidth) {
@@ -303,6 +302,10 @@ void RecordOpensOutstanding(int size) {
                               64/*num_buckets*/);
 }
 
+void RecordContiguousWriteTime(base::TimeDelta time_blocked) {
+  UMA_HISTOGRAM_TIMES("Download.FileThreadBlockedTime", time_blocked);
+}
+
 void RecordSavePackageEvent(SavePackageEvent event) {
   UMA_HISTOGRAM_ENUMERATION("Download.SavePackage",
                             event,