ReadableStream.cancel's parameter should be optional.

LayoutTests/http/tests/fetch/script-tests/stream-reader.js

Index: LayoutTests/http/tests/fetch/script-tests/stream-reader.js
diff --git a/LayoutTests/http/tests/fetch/script-tests/stream-reader.js b/LayoutTests/http/tests/fetch/script-tests/stream-reader.js
index 8e4d433c8ae1bbca427c041bd11d78915c73381d..7ec5ad803f385d225c3aa2a913a5b92aab9afed3 100644
--- a/LayoutTests/http/tests/fetch/script-tests/stream-reader.js
+++ b/LayoutTests/http/tests/fetch/script-tests/stream-reader.js
@@ -102,5 +102,16 @@ sequential_promise_test(function(t) {
       });
   }, 'Clone after reading partially');
 
+sequential_promise_test(function(t) {
+    var read = 0;

[tyoshino (SeeGerritForStatus), 2015/03/23 03:57:21]

remove

[yhirano, 2015/03/23 05:47:31]

Reverted

+    var original;
+    return fetch('/fetch/resources/progressive.php').then(function(res) {
+        res.body.cancel();
+        return res.text();
+      }).then(function(text) {
+        assert_equals(text, '');
+      });
+  }, 'Cancelling stream stops downloading.');
+
 sequential_promise_test_done();
 done();