{Request, Response}.blob() should release its lock when resolved.

LayoutTests/http/tests/fetch/script-tests/response.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 function consume(reader) {
   var chunks = [];
   function rec(reader) {
     return reader.read().then(function(r) {
         if (r.done) {
           return chunks;
@@ skipping 290 matching lines @@
 promise_test(function(t) {
     var res = new Response(new Blob(['']));
     return res.blob()
       .then(function(blob) {
           assert_equals(blob.type, '');
           assert_equals(res.headers.get('Content-Type'), null);
         });
   }, 'MIME type for Blob');
 
 promise_test(function(t) {
-    var res = new Response(new Blob([''], {type: 'Text/Plain'}));
+    var res = new Response(new Blob(['hello'], {type: 'Text/Plain'}));
     return res.blob()
       .then(function(blob) {
           assert_equals(blob.type, 'text/plain');
+          assert_equals(blob.size, 5);
+          assert_equals(res.headers.get('Content-Type'), 'text/plain');
+          return res.blob();
+        }).then(function(blob) {
+          // When we read from a response twice, it returns an empty contents.
+          // But the type should remain.
+          assert_equals(blob.type, 'text/plain');
+          assert_equals(blob.size, 0);
           assert_equals(res.headers.get('Content-Type'), 'text/plain');
         });
   }, 'MIME type for Blob with non-empty type');
 
 promise_test(function(t) {
     var res = new Response(new FormData());
     return res.blob()
       .then(function(blob) {
           assert_equals(blob.type.indexOf('multipart/form-data; boundary='),
                         0);
@@ skipping 94 matching lines @@
         assert_equals(texts[0], '');
         assert_equals(texts[1], '');
         return res.body.getReader().read();
       }).then(function(r) {
         assert_true(r.done);
         assert_equals(r.value, undefined);
       });
   }, 'Read after text()');
 
 done();