Selenium: record video of every test

Selenium: record video of every test

In addition to two existing modes (headless and visible), this patch
introcues another mode, headless recording.

If DISPLAY environment variable is not set, Chrome will run in it's
built-in headless mode.
If it is set, and the value starts with colon (`:`), Chrome will
run in headless mode using Xvfb and record video of every test using FFmpeg.
If the value does not start with colon, Chrome will be visible.

Bug: T179188
Change-Id: Ic1723c5f2d57a28201caf6ba7056cb73fb74a957