[Picture-in-Picture] Add [Exposed=Window] attribute to interfaces

This is a no-op since [Exposed=Window] is the default for interfaces. It
updates Blink's WebIDL to match the specification as modified in,

https://github.com/w3c/picture-in-picture/pull/164

Change-Id: I8aafda2233dd2e39153721e813711aec9b43afe2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1767667
Auto-Submit: François Beaufort <beaufort.francois@gmail.com>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#689944}

third_party/blink/renderer/modules/picture_in_picture/picture_in_picture_window.idl

@@ -5,6 +5,7 @@
 // https://wicg.github.io/picture-in-picture/#interface-picture-in-picture-window
 [
     ActiveScriptWrappable,
+    Exposed=Window,
     RuntimeEnabled=PictureInPictureAPI
 ]
 interface PictureInPictureWindow : EventTarget {