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

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

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 var URL = 'https://www.example.com/test.html';
 
 test(function() {
     var headers = new Headers;
     headers.set('User-Agent', 'Mozilla/5.0');
     headers.set('Accept', 'text/html');
@@ skipping 507 matching lines @@
   }, 'MIME type for Blob');
 
 promise_test(function(t) {
     var req = new Request('http://localhost/',
                           {method: 'POST',
                            body: new Blob([''], {type: 'Text/Plain'})});
     return req.blob()
       .then(function(blob) {
           assert_equals(blob.type, 'text/plain');
           assert_equals(req.headers.get('Content-Type'), 'text/plain');
-// TODO(yhirano): Currently blob() calling sets |bodyUsed| parmanently. Fix it.
-//          return new Request(req).blob();
-//        }).then(function(blob) {
-//          assert_equals(blob.type, 'text/plain');
+          return new Request(req).blob();
+        }).then(function(blob) {
+          assert_equals(blob.type, 'text/plain');
         });
   }, 'MIME type for Blob with non-empty type');
 
 promise_test(function(t) {
     var req = new Request('http://localhost/',
                           {method: 'POST', body: new FormData()});
     return req.blob()
       .then(function(blob) {
           assert_equals(blob.type.indexOf('multipart/form-data; boundary='),
                         0);
@@ skipping 69 matching lines @@
 
 // Tests for requests context.
 test(function() {
     var request = new Request('http://localhost/', {method: 'POST', body: ''});
     assert_equals(request.context, '',
                   'Request.context should be empty string ' +
                   'for synthetic Request object');
   }, 'RequestContext of a synthetic Request object');
 
 done();