Fix duplicate id in declarative Shadow DOM test

The same id was used twice, whereas they should have been unique.

No behavior changes, just a cleanup, so TBR:
TBR=masonfreed@chromium.org

Bug: 1042130
Change-Id: I58e4acc00acabbb728c730fe3910aea65c4c11e1
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601052Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839198}

third_party/blink/web_tests/external/wpt/shadow-dom/declarative/declarative-shadow-dom-opt-in.tentative.html

@@ -137,14 +137,16 @@ test(() => {
 }, 'insertAdjacentHTML on element - not supported');
 
 test(() => {
-  document.write(`<div id=doc-write-1 style="display:none">${content}</div>`);
-  assert_dsd(document.getElementById('doc-write-1'),true);
+  const id = 'doc-write-1';
+  document.write(`<div id=${id} style="display:none">${content}</div>`);
+  assert_dsd(document.getElementById(id),true);
 }, 'document.write allowed from synchronous script loaded from main document');
 
 test(() => {
+  const id = 'doc-write-2';
   const doc = document.implementation.createHTMLDocument('');
-  doc.write(`<div id=doc-write-1>${content}</div>`);
-  assert_dsd(doc.getElementById('doc-write-1'),false);
+  doc.write(`<div id=${id}>${content}</div>`);
+  assert_dsd(doc.getElementById(id),false);
 }, 'document.write disallowed on fresh document');