Change ChildNodePart comment nodes from "S"/"E" to "#" and "/"

This better matches the declarative HTML syntax, which is:

  <div parseparts>{{#}} child nodes {{/}}</div>

No real functional change, since typical usage doesn't examine the
content of these comments.

Bug: 40271855
Change-Id: Id2a4da2ee22b34e04dca2e4adca7bcacb57e5207
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5640573
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1317945}

dom/parts/basic-dom-part-declarative-brace-syntax-innerhtml.tentative.html

@@ -13,9 +13,11 @@
 </div>
 
 <script>
+const kChildNodePartStartCommentData = "#";
+const kChildNodePartEndCommentData = "/";
 function assertIsComment(node, commentText) {
   assert_true(node instanceof Comment);
-  // TODO(crbug.com/40271855): While developing alternative syntax, the comment might be empty or it might be "S"/"E".
+  // TODO(crbug.com/40271855): While developing alternative syntax, the comment might be empty or it might be "#" or "/".
   assert_true(node.textContent === '' || node.textContent === commentText);
 }
 
@@ -41,13 +43,13 @@
         let expectedRootParts = [{type:'ChildNodePart',metadata:[]}];
         assertEqualParts(root.getParts(),expectedRootParts,0,'declarative root missing parts');
         const childPart1 = root.getParts()[0];
-        assertIsComment(childPart1.previousSibling,'S');
-        assertIsComment(childPart1.nextSibling,'E');
+        assertIsComment(childPart1.previousSibling,kChildNodePartStartCommentData);
+        assertIsComment(childPart1.nextSibling,kChildNodePartEndCommentData);
         const expectedChild1Parts = [{type:'ChildNodePart',metadata:[]}];
         assertEqualParts(childPart1.getParts(),expectedChild1Parts,0,'First level childpart should just have one child part');
         const childPart2 = childPart1.getParts()[0];
-        assertIsComment(childPart2.previousSibling,'S');
-        assertIsComment(childPart2.nextSibling,'E');
+        assertIsComment(childPart2.previousSibling,kChildNodePartStartCommentData);
+        assertIsComment(childPart2.nextSibling,kChildNodePartEndCommentData);
         const expectedChild2Parts = [{type:'NodePart',metadata:[]}];
         assertEqualParts(childPart2.getParts(),expectedChild2Parts,0,'Second level childpart should have just the node part');
         assert_true(childPart2.getParts()[0].node instanceof HTMLSpanElement);

dom/parts/basic-dom-part-declarative-brace-syntax.tentative.html

@@ -55,14 +55,16 @@ <h1 id="name" parseparts>
 function addPartsCleanup(t,partRoot) {
   t.add_cleanup(() => partRoot.getParts().forEach(part => part.disconnect()));
 }
+const kChildNodePartStartCommentData = "#";
+const kChildNodePartEndCommentData = "/";
+function assertIsComment(node, commentText) {
+  assert_true(node instanceof Comment);
+  // TODO(crbug.com/40271855): While developing alternative syntax, the comment might be empty or it might be "#" or "/".
+  assert_true(node.textContent === '' || node.textContent === commentText);
+}
 
 const template = document.getElementById('declarative');
 ['Main Document','Template','Clone','PartClone'].forEach(testCase => {
-  function assertIsComment(node, commentText) {
-    assert_true(node instanceof Comment);
-    // TODO(crbug.com/40271855): While developing alternative syntax, the comment might be empty or it might be "S"/"E".
-    assert_true(node.textContent === '' || node.textContent === commentText);
-  }
   test((t) => {
     let doc,target,wrapper,cleanup;
     let expectDOMParts = true;
@@ -116,13 +118,13 @@ <h1 id="name" parseparts>
         assert_equals(root.getParts()[i].node,target.querySelector(`#nodepart${i}`));
       }
       const childPart1 = root.getParts()[0];
-      assertIsComment(childPart1.previousSibling,'S');
-      assertIsComment(childPart1.nextSibling,'E');
+      assertIsComment(childPart1.previousSibling,kChildNodePartStartCommentData);
+      assertIsComment(childPart1.nextSibling,kChildNodePartEndCommentData);
       const expectedChild1Parts = [{type:'ChildNodePart',metadata:[]}];
       assertEqualParts(childPart1.getParts(),expectedChild1Parts,0,'First level childpart should just have one child part');
       const childPart2 = childPart1.getParts()[0];
-      assertIsComment(childPart2.previousSibling,'S');
-      assertIsComment(childPart2.nextSibling,'E');
+      assertIsComment(childPart2.previousSibling,kChildNodePartStartCommentData);
+      assertIsComment(childPart2.nextSibling,kChildNodePartEndCommentData);
       const expectedChild2Parts = [{type:'NodePart',metadata:[]},{type:'AttributePart',metadata:[]}];
       assertEqualParts(childPart2.getParts(),expectedChild2Parts,0,'Second level childpart should have NodePart and AttributePart');
       assert_true(childPart2.getParts()[0].node instanceof HTMLSpanElement);