Move the anchor attribute to its own (dependent) flag

Just as a precaution, this gates the `anchor` attribute behavior
on its own `HTMLAnchorAttribute` feature, which is implied by the
`CSSAnchorPositioning` feature. This is just in case we are not
able to ship the `anchor` attribute due to delays at WHATWG:

 - whatwg/html#9144

Bug: 40059176
Change-Id: Ic4be7cc935bc9db03eba2f0477e5dfd69d8e0ba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5438893
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1285571}

third_party/blink/renderer/core/dom/element.cc

@@ -1235,13 +1235,17 @@ void Element::InterestGained() {
 Element* Element::anchorElement() const {
   // TODO(crbug.com/1425215): Fix GetElementAttribute() for out-of-tree-scope
   // elements, so that we can remove the hack below.
+  if (!RuntimeEnabledFeatures::HTMLAnchorAttributeEnabled()) {
+    return nullptr;
+  }
   if (!IsInTreeScope()) {
     return nullptr;
   }
   return GetElementAttribute(html_names::kAnchorAttr);
 }
 
 void Element::setAnchorElement(Element* new_element) {
+  CHECK(RuntimeEnabledFeatures::HTMLAnchorAttributeEnabled());
   SetElementAttribute(html_names::kAnchorAttr, new_element);
   EnsureAnchorElementObserver().Notify();
 }

third_party/blink/renderer/core/dom/element.h

@@ -1363,6 +1363,8 @@ class CORE_EXPORT Element : public ContainerNode, public Animatable {
 
   // Retrieves the element pointed to by this element's 'anchor' content
   // attribute, if that element exists.
+  // TODO(crbug.com/40059176) If the HTMLAnchorAttribute feature is disabled,
+  // this will return nullptr;
   Element* anchorElement() const;
   void setAnchorElement(Element*);
 

third_party/blink/renderer/core/dom/element.idl

@@ -143,7 +143,7 @@ dictionary CheckVisibilityOptions {
     readonly attribute long clientHeight;
 
     // Used by both Anchor Positioning and Popover
-    [CEReactions,RuntimeEnabled=CSSAnchorPositioning] attribute Element? anchorElement;
+    [CEReactions,RuntimeEnabled=HTMLAnchorAttribute] attribute Element? anchorElement;
 
     // Non-standard API
     [MeasureAs=ElementScrollIntoViewIfNeeded] void scrollIntoViewIfNeeded(optional boolean centerIfNeeded);

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2023,6 +2023,13 @@
       status: "stable",
       base_feature: "none",
     },
+    // The `anchor` attribute, supported by both anchor positioning and the
+    // popover API.
+    {
+      name: "HTMLAnchorAttribute",
+      status: "experimental",
+      implied_by: ["CSSAnchorPositioning"]
+    },
     // Adds support for the experimental `interesttarget`
     // attributes, as specified in the open-ui "Interest Invokers" explainer.
     // https://open-ui.org/components/interest-invokers.explainer/