Bug 1536306 [wpt PR 15883] - Update interfaces/pointerevents.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/pointerevents.idl (#15883)

Source: https://github.com/tidoust/reffy-reports/blob/e0218fe/whatwg/idl/pointerevents.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/506822321
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wpt-commits: 94c35acd156ab9e38b79a00ef3f333b1a03817a3
wpt-pr: 15883

testing/web-platform/tests/interfaces/pointerevents.idl

@@ -36,7 +36,7 @@ partial interface Element {
   boolean hasPointerCapture(long pointerId);
 };
 
-partial interface GlobalEventHandlers {
+partial interface mixin GlobalEventHandlers {
     attribute EventHandler ongotpointercapture;
     attribute EventHandler onlostpointercapture;
     attribute EventHandler onpointerdown;