Change BodyStreamBuffer to be FetchDataConsumerHandle-based and enable backpressure in Fetch API

LayoutTests/http/tests/workers/resources/performance-timeline-worker.js

 importScripts('../../resources/testharness.js');
 
 promise_test(function(test) {
     var durationMsec = 100;
 
     return new Promise(function(resolve) {
         performance.mark('startMark');
         setTimeout(resolve, durationMsec);
       }).then(function() {
           performance.mark('endMark');
@@ skipping 16 matching lines @@
           assert_equals(performance.getEntriesByType('measure').length, 0);
       });
   }, 'User Timing');
 
 promise_test(function(test) {
     return fetch('../../resources/dummy.txt')
       .then(function(resp) {
           return resp.text();
         })
       .then(function(t) {
+          return new Promise(function(resolve, reject) {
+              setTimeout(function() { resolve(t); }, 1000);
+            });
+        })
+      .then(function(t) {
           var expectedResources = ['/resources/testharness.js', '/resources/dummy.txt'];
           assert_equals(performance.getEntriesByType('resource').length, expectedResources.length);
           for (var i = 0; i < expectedResources.length; i++) {
               var entry = performance.getEntriesByType('resource')[i];
               assert_true(entry.name.endsWith(expectedResources[i]));
               assert_equals(entry.workerStart, 0);
               assert_greater_than(entry.startTime, 0);
               assert_greater_than(entry.responseEnd, entry.startTime);
           }
           return new Promise(function(resolve) {
               performance.onwebkitresourcetimingbufferfull = resolve;
               performance.webkitSetResourceTimingBufferSize(expectedResources.length);
             });
         })
       .then(function() {
           performance.webkitClearResourceTimings();
           assert_equals(performance.getEntriesByType('resource').length, 0);
         })
   }, 'Resource Timing');
 
 done();