Tweak the loop in ChunkedStreamManager.abort to clarify what's being iterated (PR 11985 follow-up)

In hindsight, using the `for (let [key, value] of myMap) { ... }`-format when we don't care about the `key` probably wasn't such a great idea. Since `Map`s have explicit support for iterating either `key`s or `value`s, we should probably use that instead here.

src/core/chunked_stream.js

@@ -591,7 +591,7 @@ class ChunkedStreamManager {
     if (this.pdfNetworkStream) {
       this.pdfNetworkStream.cancelAllRequests(reason);
     }
-    for (const [, capability] of this._promisesByRequest) {
+    for (const capability of this._promisesByRequest.values()) {
       capability.reject(reason);
     }
   }