[Fetch] Support various operations after reading data partially.

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

 if (self.importScripts) {
   importScripts('/fetch/resources/fetch-test-helpers.js');
 }
 
 function read_until_end(reader) {
   var chunks = [];
   function consume() {
     return reader.read().then(function(r) {
         if (r.done) {
           return chunks;
@@ skipping 57 matching lines @@
         res.body;
         assert_false(res.bodyUsed);
         res.text();
         assert_true(res.bodyUsed);
         // FIXME: Getting a reader should throw, but it doesn't because the
         // current implementation closes the body.
         // assert_throws({name: 'TypeError'}, function() { res.body.getReader() });
       });
   }, 'Setting bodyUsed means the body is locked.');
 
+sequential_promise_test(function(t) {
+    var reader;
+    var read = 0;
+    var original;
+    return fetch('/fetch/resources/progressive.php').then(function(res) {
+        original = res;
+        reader = res.body.getReader();
+        return reader.read();
+      }).then(function(r) {
+        assert_false(r.done);
+        read += r.value.byteLength;
+        // Make sure that we received something but we didn't receive all.
+        assert_not_equals(read, 0);
+        assert_not_equals(read, 190);
+
+        reader.releaseLock();
+        return read_until_end(original.clone().body.getReader());
+      }).then(function(chunks) {
+        for (var chunk of chunks) {
+          read += chunk.byteLength;
+        }
+        // Make sure that we received all data in total.
+        assert_equals(read, 190);
+      });
+  }, 'Clone after reading partially');
+
 sequential_promise_test_done();
 done();