[Picture-in-Picture] Fix web platform tests that time out.

This makes sure web platform tests for Picture-in-Picture do not time
out when Picture-in-Picture API is not available.

Besides, wpt/picture-in-picture/request-picture-in-picture is split
to work around a flaky timeout in WPT upstream that's related to
https://github.com/web-platform-tests/wpt/issues/10398

Note: based on https://github.com/web-platform-tests/wpt/pull/11329

Bug: 806249
Change-Id: I6185253dfa4aae5111e87553581b8464032558d1
Reviewed-on: https://chromium-review.googlesource.com/1107626
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Fran├žois Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#576804}
7 files changed