[WebGL] WebGL is not working

The following works as expected in Qt 5.10.1, but not on Qt 5.11 Beta2.

Steps to reproduce the issue:
1. Run photoviever example (or other Qt Quick application) with parameter "-platform webgl:port=1234"

2. Direct browser to address http://localhost:1234/

Expected outcome:
The GUI should be visibile in browser window.

Actual outcode:
Moving green circle is visible for a while and after that just black grey screen is visible.