[Nightly] YouTube videos are not launched after clicking the Min Vid overlay

[SoftVision-CarmenFat]

[Notes]:

• This will be a blocker if the launch of the study is postponed enough until today's Nightly 61.0a1 will become Release 61.
• This is a regression introduced by issue 1446585.
• Last good revision: 3518de30af30f44ba9656722705a580de3012616
• First bad revision: 6f9a0b898e9367f2590685e683dbb2bcbe61ba6b
• Pushlog: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=3518de30af30f44ba9656722705a580de3012616&tochange=6f9a0b898e9367f2590685e683dbb2bcbe61ba6b

[Affected versions]:

• Firefox Nightly 61.0a1 (04/05/2018)
• Min-Vid Shield Study Onboarding 0.4.7 - Shield Study

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Latest Min-Vid v.0.4.7 Shield Study Onboarding ("active" or "activeAndOnboarding" variant) was previously installed on a clean profile.

[Steps to reproduce]:

1. Open latest Nightly with the profiles from prerequisites and navigate to YouTube.
2. Click the Min Vid overlay and observe what happens next.

[Expected result]:

• The selected video starts playback.

[Actual result]:

• Nothing happens, the Min Vid player is not launched.

[Additional notes]:

• Here is a screen recording of the issue:

[ghost]

Still an issue in Nightly 62.0a1 (Linux)