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

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

Index: LayoutTests/http/tests/workers/resources/performance-timeline-worker.js
diff --git a/LayoutTests/http/tests/workers/resources/performance-timeline-worker.js b/LayoutTests/http/tests/workers/resources/performance-timeline-worker.js
index f47c3b357af2d45b03a8664253aa6392e2e55dde..86f47b2a56a2ae84423d052db6e8eb4d5094cee8 100644
--- a/LayoutTests/http/tests/workers/resources/performance-timeline-worker.js
+++ b/LayoutTests/http/tests/workers/resources/performance-timeline-worker.js
@@ -34,6 +34,11 @@ promise_test(function(test) {
           return resp.text();
         })
       .then(function(t) {
+          return new Promise(function(resolve, reject) {

[hiroshige, 2015/07/02 08:22:10]

This is temporary fix: |performance| entries are updated almost the same time
when .text() is resolved, but not necessarily before .text() is resolved. So I
inserted a sleep, but I'll resolve this issue next week with ksakamoto@.

+              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++) {