[ServiceWorker] Add support of FetchEvent's request.body.

Source/modules/serviceworkers/Request.idl

Index: Source/modules/serviceworkers/Request.idl
diff --git a/Source/modules/serviceworkers/Request.idl b/Source/modules/serviceworkers/Request.idl
index 3d1b402ae608e81b4fd56bdfd460e5864559a434..964984f5c7297564c65d402c9fd9e1edc30ab659 100644
--- a/Source/modules/serviceworkers/Request.idl
+++ b/Source/modules/serviceworkers/Request.idl
@@ -19,12 +19,12 @@ enum RequestCredentials { "omit", "same-origin", "include" };
     readonly attribute ByteString method;
     readonly attribute ScalarValueString url;
     readonly attribute Headers headers;
+    [CallWith=ExecutionContext] readonly attribute FetchBodyStream body;
 
     readonly attribute DOMString referrer;
     readonly attribute RequestMode mode;
     readonly attribute RequestCredentials credentials;
 
     // FIXME: Implement the following:
-    // readonly attribute FetchBodyStream body;
     // readonly attribute RequestContext context;
 };