Bug 1523562 [wpt PR 14990] - Update interfaces/picture-in-picture.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/picture-in-picture.idl (#14990)

Source: https://github.com/tidoust/reffy-reports/blob/8a0d273/whatwg/idl/picture-in-picture.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/482751671
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wpt-commits: 3ed3e070163bd165132913887da804d87da159c2
wpt-pr: 14990

testing/web-platform/tests/interfaces/picture-in-picture.idl

@@ -18,7 +18,7 @@ partial interface Document {
   [NewObject] Promise<void> exitPictureInPicture();
 };
 
-partial interface DocumentOrShadowRoot {
+partial interface mixin DocumentOrShadowRoot {
   readonly attribute Element? pictureInPictureElement;
 };