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

LayoutTests/http/tests/cachestorage/script-tests/cache-put.js

 if (self.importScripts) {
     importScripts('/resources/testharness.js');
     importScripts('/resources/testharness-helpers.js');
     importScripts('../resources/test-helpers.js');
 }
 
 var test_url = 'https://example.com/foo';
 var test_body = 'Hello world!';
 
 cache_test(function(cache) {
@@ skipping 273 matching lines @@
                  'Response.bodyUsed should be initially false.');
     return response.text().then(function() {
       assert_false(
         response.bodyUsed,
         '[https://fetch.spec.whatwg.org/#concept-body-consume-body] ' +
           'The text() method should not set "body passed" flag.');
       return cache.put(new Request(test_url), response);
     });
   }, 'Cache.put with a used response body');
 
+cache_test(function(cache) {
+    var response = new Response(test_body);
+    return cache.put(new Request(test_url), response)
+      .then(function() {
+          return response.body.getReader().closed;
+      });
+  }, 'getReader() after Cache.put');
+
 done();