Fix cancelPromise in ReadableStreamTee being resolved twice (again)

Browsers were not handling this case correctly (i.e. ignoring the second resolve), as can be seen from web-platform-tests/wpt#28266.

Author: MattiasBuelens

index.bs

@@ -2240,7 +2240,7 @@ create them does not matter.
        [$ReadableStreamDefaultControllerClose$](|branch1|.[=ReadableStream/[[controller]]=]).
     1. If |canceled2| is false, perform !
        [$ReadableStreamDefaultControllerClose$](|branch2|.[=ReadableStream/[[controller]]=]).
-    1. [=Resolve=] |cancelPromise| with undefined.
+    1. If |canceled1| is false or |canceled2| is false, [=resolve=] |cancelPromise| with undefined.
 
    : [=read request/error steps=]
    ::

reference-implementation/lib/abstract-ops/readable-streams.js

@@ -373,7 +373,9 @@ function ReadableStreamTee(stream, cloneForBranch2) {
         if (canceled2 === false) {
           ReadableStreamDefaultControllerClose(branch2._controller);
         }
-        resolvePromise(cancelPromise, undefined);
+        if (canceled1 === false || canceled2 === false) {
+          resolvePromise(cancelPromise, undefined);
+        }
       },
       errorSteps: () => {
         reading = false;