Fix exception when trying to reference this inside callback (problem introduced by #5217)

[diarmidmackenzie]

Description:

I recently proposed PR#5217 & it was merged. Unfortunately it triggers an exception on entry to WebXR, and also doesn't do what it was supposed to do.

This fixes that

Thanks to @mikemainguy for spotting this. He made a comment in #5217 yesterday, although I now can't find that comment, so maybe he since deleted it?

Changes proposed:

Originally submitted code was fatally flawed as it assumed that this was bound to the element within a callback.

I've corrected the code by using the self variable instead.

This is a complete screw-up by me. Sorry. Full disclosure on how this happened, so we (most of all me) can learn from this.

• Prior to testing, I had built an external component that interacted with the WebXR API, and tested that extensively live, so I was confident I knew how the API worked.
• Original code submitted for the PR was explicitly untested - intended for discussion about the feature
• I then refactored the code, and did a full Unit Test of the code in the renderer component
• I didn't UT the code in a-scene because the existing UTs in this area were limited, meaning it would have been a lot of work to extend them to cover this case. And since it was "only one line", I told myself the risk was low. If I had done a proper UT, I would have found the problem.
• I also didn't test live at this point. If I had tested live and monitored the console output, I would also have found the problem.

Since master is broken right now (though the impact is just a console error, no other functional issues other than 5217 not actually working), I propose merging this fix now. I have tested this live on a Quest 2, including stepping through the code in the Chrome tools debugger.

Separately I will do a PR to extend coverage of the UTs for entering VR in WebXR mode (they currently focus on the legacy WebVR case).

[dmarcos]

Thanks, good catch! Sorry this bug slipped in.