Add methods for creating V8 extras-based ReadableStreams from C++

third_party/WebKit/Source/core/streams/UnderlyingSourceBase.idl

Index: third_party/WebKit/Source/core/streams/UnderlyingSourceBase.idl
diff --git a/third_party/WebKit/Source/core/streams/UnderlyingSourceBase.idl b/third_party/WebKit/Source/core/streams/UnderlyingSourceBase.idl
new file mode 100644
index 0000000000000000000000000000000000000000..f0ec1b76932537bdc63a505b43158d40305eb3de
--- /dev/null
+++ b/third_party/WebKit/Source/core/streams/UnderlyingSourceBase.idl
@@ -0,0 +1,15 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This is not a spec interface. Instead, it is used to generate bindings
+// so that Blink code which wishes to create a stream can derive from
+// UnderlyingSourceBase, and a JavaScript object can then be generated
+// automatically for use in initializing a ReadableStream.
+
+[NoInterfaceObject, GarbageCollected, ActiveDOMObject]
+interface UnderlyingSourceBase {
+    [CallWith=ScriptState, ImplementedAs=startWrapper] Promise<void> start(any stream);
+    [CallWith=ScriptState] Promise<void> pull();
+    [CallWith=ScriptState, ImplementedAs=cancelWrapper] Promise<void> cancel([Default=Undefined] optional any reason);
+};