SceneReader sets contentSize by canvasSize.

We could not get the canvas size of the scene which created on CocosStudio.

Thanks to this commit, we can get scene's canvas size by Node::getContentSize()

```cpp
auto scene = cocos2d::cocostudio::SceneReader::getInstance()->createNodeWithSceneFile("Scene.json");
int width = scene->getContentSize().width;
int height = scene->getContentSize().height;
cocos2d::log("canvasSize = %d, %d", width, height);
```
Merge pull request #7979 from giginet/scene-reader-canvas-size

SceneReader sets contentSize by canvasSize.