[css-pseudo] Hide newly created ::marker behind flag in devtools tree

The ::marker pseudo-element is being implemented behind a flag, so it
shouldn't be exposed in devtools unless this flag is enabled.

That was already happening for ::markers that already existed before
opening the devtools tree. However, if a ::marker was created
afterwards (e.g. by setting 'display: list-item' to some element), then
the ::marker was shown even if the flag was disabled.

This patch fixes the issue so that ::marker is only shown when the flag
is enabled.

BUG=457718

Change-Id: Ie2261c15a1211599355ca687b38cc525be8e2bb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027909
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736575}

third_party/blink/renderer/core/inspector/inspector_dom_agent.cc

@@ -2083,6 +2083,10 @@ void InspectorDOMAgent::FrameOwnerContentUpdated(
 }
 
 void InspectorDOMAgent::PseudoElementCreated(PseudoElement* pseudo_element) {
+  if (pseudo_element->IsMarkerPseudoElement() &&
+      !RuntimeEnabledFeatures::CSSMarkerPseudoElementEnabled()) {
+    return;
+  }
   Element* parent = pseudo_element->ParentOrShadowHostElement();
   if (!parent)
     return;