Picture-in-Picture: disable :picture-in-picture CSS pseudo-class.

We would like to skip M76 as we may need to revisit the design based
on new features coming into the Picture-in-Picture API.

Bug: 940404
Change-Id: Iec0912a249fd0fbbed16103548aaa64fb0842921
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632119Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663969}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -375,7 +375,7 @@
     },
     {
       name: "CSSPictureInPicture",
-      status: "stable",
+      status: "experimental",
       depends_on: ["PictureInPictureAPI"],
     },
     {