Unity3d FullScreen Mode Vertical Aspect Ratio in WebGl

my game has vertical aspect ratio and looks like this:

Code (JavaScript):

1. When i press full screen button
2.  
3. <div class="webgl-content">
4.     <div id="gameContainer" style="width: 500px; height: 860px"></div>
5. </div>
6. <div class="fullscreen" onclick="gameInstance.SetFullscreen(1)"></div>

My game start to look like this:

How to save aspect ratio? Thank you