Media controls show for non-video sources in VR video modes

[caseyyee]

FXR:
eaa4fc5

Hardware

Oculus Go

Steps to Reproduce

1. Enter fullscreen
2. Navigate to
   https://mixedreality.mozilla.org/fxr-html-tests/window-size.html
3. Fullscreen
4. Select VR video mode

Current Behavior

Media transport controls show. If you interact with the controls, the browser crashes.

Expected Behavior

For non-video sources, we should not be showing transport controls.

[cvan]

@caseyyee I recall you saying you saw some crashes with non-video sources? can you provide STR?

[cvan]

From my testing, this is important because it is confusing to the user, but it does not block a user flow or crash. @caseyyee if you can reproduce with a crash, please post a comment with adb logcat | grep -i gecko logs, and we can escalate this issue. Thanks!

[MortimerGoro]

I created #796 for now

@thenadj created a different UI for this scenario, but it's too much work for v1.1. We can address this later