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

Source/bindings/core/v8/ScriptState.cpp

Index: Source/bindings/core/v8/ScriptState.cpp
diff --git a/Source/bindings/core/v8/ScriptState.cpp b/Source/bindings/core/v8/ScriptState.cpp
index 01756b8fef38a3cde3c4d52170a2bc68ae037a8f..911daa01a2990063eb6a89095be6139af851e5f6 100644
--- a/Source/bindings/core/v8/ScriptState.cpp
+++ b/Source/bindings/core/v8/ScriptState.cpp
@@ -102,6 +102,15 @@ ScriptValue ScriptState::getFromGlobalObject(const char* name)
     return ScriptValue(this, v8Value);
 }
 
+ScriptValue ScriptState::getFromExtrasExports(const char* name)
+{
+    v8::HandleScope handleScope(m_isolate);
+    v8::Local<v8::Value> v8Value;
+    if (!context()->GetExtrasExportsObject()->Get(context(), v8AtomicString(isolate(), name)).ToLocal(&v8Value))
+        return ScriptValue();
+    return ScriptValue(this, v8Value);
+}
+
 ExecutionContext* ScriptState::executionContext() const
 {
     v8::HandleScope scope(m_isolate);