Include ytd-video-masthead-ad to removeEmptyElements.css

[luixxiul]

Addresses #7818

Auditors: @bbondy @lukemulks

Test Plan:

1. Open youtube.com in the experimental mode (try on the privade tab)
2. Make sure you do not see the div placeholder

• 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).

[lukemulks]

@luixxiul @bsclifton @bbondy

Confirming that I'm not seeing the placeholder div displaying in-page:

I am seeing the expanded state css class for the element, but the placeholder size (height and width) are not displaying that matches the element in the screenshot from the original issue (which was an empty container, expanded state, included a click to close ad button).

Confirming that the ad call for the 970x250 masthead ad is blocked

I did check against the production version of the page, and I'm not seeing the placeholder there either.
This issue might be something only users outside of the US are experiencing, because I haven't been seeing the issue before the testing today.

We might want to have someone outside of the US check this to confirm the container that @luixxiul has been seeing is no longer displaying w/this fix so we can fully cover off.

@bbondy were you seeing the empty container from your geo?

If you are/were, would you mind taking a look to confirm?

[luixxiul]

This issue might be something only users outside of the US are experiencing

I rarely see the placeholder even in Japan. The top page might be experimental one.

[cndouglas]

Should this be included in 0.15.2?

[luixxiul]

for me It is OK to close this. I no longer see the experimental page.

[bsclifton]

Closing per comment by @luixxiul