Add use counter for Form Associated Custom Elements

Bug: 905922
Change-Id: I0d2331aac15930b21c8ff565d4856050a2aa9ed2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346968
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796623}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2671,6 +2671,8 @@ enum WebFeature {
   kInlineOverflowScrollWithInlineEndPadding = 3336,
   kCSSSelectorPseudoWebKitDetailsMarker = 3337,
   kSerialPortGetInfo = 3338,
+  // The above items are available in M85 branch.
+
   kFileSystemPickerMethod = 3339,
   kV8Window_ShowOpenFilePicker_Method = 3340,
   kV8Window_ShowSaveFilePicker_Method = 3341,
@@ -2699,6 +2701,7 @@ enum WebFeature {
   kConversionRegistration = 3367,
   kWebSharePolicyAllow = 3368,
   kWebSharePolicyDisallow = 3369,
+  kFormAssociatedCustomElement = 3370,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/html/custom/custom_element_registry.cc

@@ -212,6 +212,11 @@ CustomElementDefinition* CustomElementRegistry::DefineInternal(
   NameIdMap::AddResult result = name_id_map_.insert(descriptor.GetName(), id);
   CHECK(result.is_new_entry);
 
+  if (definition->IsFormAssociated()) {
+    if (Document* document = owner_->document())
+      UseCounter::Count(*document, WebFeature::kFormAssociatedCustomElement);
+  }
+
   HeapVector<Member<Element>> candidates;
   CollectCandidates(descriptor, &candidates);
   for (Element* candidate : candidates)

tools/metrics/histograms/enums.xml

@@ -28693,6 +28693,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3367" label="ConversionRegistration"/>
   <int value="3368" label="WebSharePolicyAllow"/>
   <int value="3369" label="WebSharePolicyDisallow"/>
+  <int value="3370" label="FormAssociatedCustomElement"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">