[Fetch API] Fix leaky layout tests.

third_party/WebKit/LayoutTests/http/tests/fetch/script-tests/body-mixin.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
   importScripts('/streams/resources/rs-utils.js');
 }
 
 function isLocked(stream) {
   try {
     var reader = stream.getReader();
     reader.releaseLock();
     return false;
@@ skipping 242 matching lines @@
 promise_test(t => {
     var res = new Response('');
     res.body.cancel();
     return res.text().then(unreached_fulfillment(t), e => {
         assert_equals(e.name, 'TypeError');
       });
   }, 'Used => text');
 
 promise_test(t => {
     var res = new Response('');
-    res.body.getReader();
+    const reader = res.body.getReader();
     return res.arrayBuffer().then(unreached_fulfillment(t), e => {
+        // TODO: yhirano Use finally when it's available. Ditto below.

[Yuki, 2016/04/25 06:57:10]

typo: TODO(yhirano): ...

[yhirano, 2016/04/25 07:01:51]

Done.

+        reader.releaseLock();
         assert_equals(e.name, 'TypeError');
       });
   }, 'Locked => arrayBuffer');
 
 promise_test(t => {
     var res = new Response('');
-    res.body.getReader();
+    const reader = res.body.getReader();
     return res.blob().then(unreached_fulfillment(t), e => {
+        reader.releaseLock();
         assert_equals(e.name, 'TypeError');
       });
   }, 'Locked => blob');
 
 promise_test(t => {
     var res = new Response('');
-    res.body.getReader();
+    const reader = res.body.getReader();
     return res.json().then(unreached_fulfillment(t), e => {
+        reader.releaseLock();
         assert_equals(e.name, 'TypeError');
       });
   }, 'Locked => json');
 
 promise_test(t => {
     var res = new Response('');
-    res.body.getReader();
+    const reader = res.body.getReader();
     return res.text().then(unreached_fulfillment(t), e => {
+        reader.releaseLock();
         assert_equals(e.name, 'TypeError');
       });
   }, 'Locked => text');
 
 done();