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

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

Index: LayoutTests/http/tests/cachestorage/script-tests/cache-put.js
diff --git a/LayoutTests/http/tests/cachestorage/script-tests/cache-put.js b/LayoutTests/http/tests/cachestorage/script-tests/cache-put.js
index b9250956c5a0c74f1be7354f48288001ac6acbc3..60d142d6fa18da88799f5944870e573f7371c80e 100644
--- a/LayoutTests/http/tests/cachestorage/script-tests/cache-put.js
+++ b/LayoutTests/http/tests/cachestorage/script-tests/cache-put.js
@@ -291,4 +291,12 @@ cache_test(function(cache) {
     });
   }, '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();