Remove use of Set#firstValue in layout test

https://crrev.com/c/2622193 removes Set#firstValue from the DevTools
utils as it is not used in the codebase - this layout test is the only
place to use it across Chromium, so I've rewritten it to inline the
functionality.

Bug: chromium:1050549
Change-Id: I86093d63aa9cf086fbc06092f8dcc4eb7c4b86f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623928
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842482}

third_party/blink/web_tests/http/tests/devtools/elements/styles-1/edit-resource-referred-by-multiple-styletags.js

@@ -56,6 +56,6 @@
       return;
     }
     TestRunner.addResult('Both headers and uiSourceCode content:');
-    TestRunner.addResult(dedup.firstValue());
+    TestRunner.addResult(dedup.size ? dedup.values().next().value : null);
   }
 })();