Use ByteStream in downloads system to decouple source and sink.

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 94bc2e04151de2710e0869611295b37b82828872..7c1054b678916491945355aee2c46a76f49c2df2 100644
--- a/content/browser/download/download_stats.h
+++ b/content/browser/download/download_stats.h
@@ -16,6 +16,7 @@
 
 namespace base {
 class Time;
+class TimeDelta;
 class TimeTicks;
 }
 
@@ -137,6 +138,9 @@ void RecordClearAllSize(int size);
 // Record the number of completed unopened downloads when a download is opened.
 void RecordOpensOutstanding(int size);
 
+// Record how long we block the file thread at a time.
+void RecordContiguousWriteTime(base::TimeDelta time_blocked);
+
 enum SavePackageEvent {
   // The user has started to save a page as a package.
   SAVE_PACKAGE_STARTED,