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

Source/core/streams/UnderlyingSourceBase.cpp

+// Copyright 2015 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.
+
+#include "config.h"
+#include "core/streams/UnderlyingSourceBase.h"
+
+#include "core/streams/ReadableStreamController.h"
+#include <v8.h>
+
+namespace blink {
+
+ScriptPromise UnderlyingSourceBase::startWrapper(ScriptState* scriptState, ScriptValue stream)
+{
+    // Cannot call start twice (e.g., cannot use the same UnderlyingSourceBase to construct multiple streams)
+    ASSERT(!m_controller);
+
+    // In ReadableStream.js, we special-case externally-controlled streams by having them pass themselves to start
+    // as the first argument. This allows us to create a ReadableStreamController.
+
+    m_controller = new ReadableStreamController(stream);
+
+    return start(scriptState);
+}
+
+ScriptPromise UnderlyingSourceBase::start(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::pull(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::cancelWrapper(ScriptState* scriptState, ScriptValue reason)
+{
+    m_controller->noteHasBeenCanceled();
+    return cancel(scriptState, reason);
+}
+
+ScriptPromise UnderlyingSourceBase::cancel(ScriptState* scriptState, ScriptValue reason)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+bool UnderlyingSourceBase::hasPendingActivity() const
+{
+    return m_controller && m_controller->isActive();
+}
+
+
+DEFINE_TRACE(UnderlyingSourceBase)
+{
+    ActiveDOMObject::trace(visitor);
+    visitor->trace(m_controller);
+}
+
+} // namespace blink