[A11y] Speculative fix for placeholder elements trying to add children

The crash reports are all logging placeholder elements as trying to add
children but having a return value of false for CanHaveChildren(). A
possible explanation is that CanHaveChildren() returned true earlier
because the placeholder element.ShadowPseudoId() value was not set
for the initial call.

This hopefully fixes the issue by using a more robust API to check for
the placeholder element. Even if it does not fix the crash, it seems
cleaner and less brittle.

Bug: 1407397
Change-Id: I59a10338525076e756e5a60cd475bdb26e75af9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4590613
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1154087}

third_party/blink/renderer/modules/accessibility/ax_object.cc

@@ -1011,9 +1011,13 @@ bool AXObject::CanHaveChildren(Element& element) {
   // so there's no need to add its text children. Placeholder text is a separate
   // node that gets removed when it disappears, so this will only be present if
   // the placeholder is visible.
-  if (element.ShadowPseudoId() ==
-      shadow_element_names::kPseudoInputPlaceholder) {
-    return false;
+  if (Element* host = element.OwnerShadowHost()) {
+    if (auto* ancestor_input = DynamicTo<TextControlElement>(host)) {
+      if (ancestor_input->PlaceholderElement() == &element) {
+        // |element| is a placeholder.
+        return false;
+      }
+    }
   }
 
   if (IsA<HTMLBRElement>(element)) {