Use pseudo-element for audio indicator

[jonathansampson]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Resolves #8033

Test Plan:

1. Navigate to https://www.youtube.com/watch?v=WsSPW2oC7pI
2. Open enough tabs to have 10 on-screen and reduce window-width to reveal player
3. Toggle video pause/play state
4. Tab contents should not shift when displaying the audio indicator

[cezaraugusto]

much better, thanks for taking that +++