Bug 1619716 [wpt PR 22070] - Update interfaces/construct-stylesheets.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/construct-stylesheets.idl (#22070)

Note: This file was recently manually updated in commit 20ecc32c04.
      This commit may revert some of those changes.

Source: https://github.com/tidoust/reffy-reports/blob/8e14b19/ed/idl/construct-stylesheets.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/657881683
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wpt-commits: 96e35970eb10bf210b782e8512a3d45ccb3d1e4c
wpt-pr: 22070

testing/web-platform/tests/interfaces/construct-stylesheets.idl

@@ -16,6 +16,6 @@ dictionary CSSStyleSheetInit {
   boolean disabled = false;
 };
 
-partial interface DocumentOrShadowRoot {
+partial interface mixin DocumentOrShadowRoot {
   attribute FrozenArray<CSSStyleSheet> adoptedStyleSheets;
 };