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

Source/core/streams/ReadableStream2.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/ReadableStream2.h"
+
+#include "bindings/core/v8/ToV8.h"
+#include "bindings/core/v8/V8RecursionScope.h"
+#include "core/streams/UnderlyingSourceBase.h"
+#include <v8.h>
+
+namespace blink {
+
+namespace JSStreams {
+
+ScriptValue createReadableStream(ScriptState* scriptState, UnderlyingSourceBase* underlyingSource, ScriptValue strategy)
+{
+    ScriptState::Scope scope(scriptState);
+    auto global = scriptState->context()->Global();
+    auto isolate = scriptState->isolate();
+
+    auto jsUnderlyingSource = toV8(underlyingSource, global, isolate);
+    auto jsStrategy = strategy.v8Value();
+
+    auto sentinel = scriptState->getFromExtrasExports("createWithExternalControllerSentinel").v8Value();
+    auto constructor = scriptState->getFromExtrasExports("ReadableStream").v8Value().As<v8::Function>();
+
+    v8::Local<v8::Value> args[] = { jsUnderlyingSource, jsStrategy, sentinel };
+
+    V8RecursionScope::MicrotaskSuppression mtsScope(isolate);
+    auto jsStream = constructor->NewInstance(arraysize(args), args);
+
+    return ScriptValue(scriptState, jsStream);
+}
+
+ScriptValue createCountQueuingStrategy(ScriptState* scriptState, size_t highWaterMark)
+{
+    ScriptState::Scope scope(scriptState);
+    auto isolate = scriptState->isolate();
+
+    auto constructor = scriptState->getFromExtrasExports("BuiltInCountQueuingStrategy").v8Value().As<v8::Function>();
+
+    v8::Local<v8::Value> args[] = { v8::Number::New(isolate, highWaterMark) };
+
+    V8RecursionScope::MicrotaskSuppression mtsScope(isolate);
+    auto jsStrategy = constructor->NewInstance(arraysize(args), args);
+
+    return ScriptValue(scriptState, jsStrategy);
+}
+
+} // namespace JSStreams
+
+} // namespace blink