[Picture-in-Picture] Update virtual tests READMEs.

This removes PictureInPicture feature in README as it is not needed for
these virtual tests.

Bug: 806249
Change-Id: Ie78767cc7261f7b2538f9849e3a2073261e091b6
Reviewed-on: https://chromium-review.googlesource.com/1092533
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565762}
2 files changed