Add use counters for delegatesFocus

The focus delegation behavior might change as proposed in
https://github.com/w3c/webcomponents/issues/830.

This CL adds use counter for when a host with delegatesFocus is focused,
and when the result of the focus delegation is different than the
proposed change, to see the possible impact of the change.

Bug: 692678
Change-Id: I90a0a10e1e5adbd3fe018ea772e1d560fedec9a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1794945Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695100}

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

@@ -2410,6 +2410,8 @@ enum WebFeature {
   kPointerLockUnadjustedMovement = 3027,
   kCreateObjectBlob = 3028,
   kQuotaRead = 3029,
+  kDelegateFocus = 3030,
+  kDelegateFocusNotFirstInFlatTree = 3031,
 
   // 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/dom/element.cc

@@ -3740,6 +3740,7 @@ void Element::focus(const FocusParams& params) {
     return;
 
   if (AuthorShadowRoot() && AuthorShadowRoot()->delegatesFocus()) {
+    UseCounter::Count(GetDocument(), WebFeature::kDelegateFocus);
     Element* focused_element = GetDocument().FocusedElement();
     if (focused_element &&
         IsShadowIncludingInclusiveAncestorOf(*focused_element))

third_party/blink/renderer/core/input/mouse_event_manager.cc

@@ -649,6 +649,7 @@ bool MouseEventManager::SlideFocusOnShadowHostIfNecessary(
   if (element.AuthorShadowRoot() &&
       element.AuthorShadowRoot()->delegatesFocus()) {
     Document* doc = frame_->GetDocument();
+    UseCounter::Count(doc, WebFeature::kDelegateFocus);
     Element* focused_element = doc->FocusedElement();
     if (focused_element &&
         element.IsShadowIncludingInclusiveAncestorOf(*focused_element)) {

third_party/blink/renderer/core/page/focus_controller.cc

@@ -1190,8 +1190,29 @@ Element* FocusController::FindFocusableElementInShadowHost(
   OwnerMap owner_map;
   ScopedFocusNavigation scope =
       ScopedFocusNavigation::OwnedByShadowHost(shadow_host, owner_map);
-  return FindFocusableElementAcrossFocusScopes(kWebFocusTypeForward, scope,
-                                               owner_map);
+  Element* result = FindFocusableElementAcrossFocusScopes(kWebFocusTypeForward,
+                                                          scope, owner_map);
+  if (!result)
+    return nullptr;
+  // Check if |found| is the first focusable element under |element|, and count
+  // if it's not.
+  const Node* current = &shadow_host;
+  while ((current = FlatTreeTraversal::Next(*current))) {
+    if (!current->IsElementNode())
+      continue;
+    if (current == result) {
+      // We've reached |found|, which means |found| is the first focusable
+      // element so we don't count this.
+      break;
+    }
+    if (ToElement(current)->IsFocusable()) {
+      UseCounter::Count(shadow_host.GetDocument(),
+                        WebFeature::kDelegateFocusNotFirstInFlatTree);
+      break;
+    }
+  }
+
+  return result;
 }
 
 Element* FocusController::FindFocusableElementAfter(Element& element,

tools/metrics/histograms/enums.xml

@@ -24615,6 +24615,8 @@ Called by update_net_error_codes.py.-->
   <int value="3027" label="PointerLockUnadjustedMovement"/>
   <int value="3028" label="CreateObjectBlob"/>
   <int value="3029" label="QuotaRead"/>
+  <int value="3030" label="DelegateFocus"/>
+  <int value="3031" label="DelegateFocusNotFirstInFlatTree"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">