Fix UIButton setFontSize sync ContentSize

Similar to this issue: https://github.com/cocos2d/cocos2d-x/issues/8567#issuecomment-58606780

Note: I didn't add a test case to this commit, but here is a simple one to reproduce:

ui::Button *button = ui::Button::create();
button->setTitleText("some text");
button->setTitleFontSize(30.0f); // set font size
Size c = button->getContentSize();
Size r = button->getVirtualRendererSize();
CCLOG("content size: %f x %f | %f x %f", c.width, c.height, r.width, r.height); // c != r

Also, it seems that since we are adding this setContentSize check in multiple locations that UIButton might need to be refactored in the future.