Use ByteStream in downloads system to decouple source and sink.

content/browser/download/byte_stream.h

Index: content/browser/download/byte_stream.h
diff --git a/content/browser/download/byte_stream.h b/content/browser/download/byte_stream.h
index 8265b8dd20e241885ceba8918e0bbfa745d6a4f0..d4c5b0bc768c84e7957e0e7a2f51edeac9fe27b7 100644
--- a/content/browser/download/byte_stream.h
+++ b/content/browser/download/byte_stream.h
@@ -82,6 +82,7 @@ public:
   // is the callers responsibility to handle races with space becoming
   // available (i.e. in the case of that race either of the before
   // or after callbacks may be called).
+  // The callback will not be called after ByteStreamInput destruction.
   virtual void RegisterCallback(const base::Closure& source_callback) = 0;
 };
 
@@ -109,6 +110,7 @@ class CONTENT_EXPORT ByteStreamOutput {
   // though note that it is the callers responsibility to handle races
   // with data becoming available (i.e. in the case of that race
   // either of the before or after callbacks may be called).
+  // The callback will not be called after ByteStreamOutput destruction.
   virtual void RegisterCallback(const base::Closure& sink_callback) = 0;
 };