Playing video can be heard playing after closing Custom Tab

[jonalmeida]

Steps to reproduce

1. Have a site with a video on a page in a Custom Tab (e.g. Use https://streamja.com/gQXZ with the CT Test app.)
2. Play the video (keep sound audible).
3. Press the close button of the CT (not the system back button).
4. Observe the audio from the playing video.

Expected behavior

• The audio should stop indicating that the video was stopped and the tab is closed.

Actual behavior

• The audio continues to play and there is no way to get back to that session again.

Device information

• Android device: Pixel 2 (Android 9)
• Reference Browser version: Nightly (15-03-19)

Other Notes

• This might be related to what we were seeing in Clicking a link in Twitter doesn't always show the website #558.
• Pressing the system back button does show the correct behaviour, which means we may need to duplicate that same functionality on the close button as well.

[jonalmeida]

a-c issue is fixed.