[Picture-in-Picture] Remove shadow DOM support for pseudo class

After intent to ship was sent, it was decided that :picture-in-picture
should not automatically apply to the shadow host chain.

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/X-qPSmdSR_g

Bug: 940404
Change-Id: I81866b58774ccfb0bc171338078af105e2bc9ab2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575578
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#655141}