Add a use counter for getComputedStyle outside the flat tree.

We would like to return empty styles for elements which are connected but
not part of the flat tree. That is, shadow host children which are not
slotted or any nodes in its flat subtree.

Bug: 831568
Change-Id: Ifd6ab89196a83571044ca995198e3e31c904304e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916460
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715234}

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

@@ -2469,6 +2469,7 @@ enum WebFeature {
   kModuleDedicatedWorker = 3085,
   kFetchBodyStreamInServiceWorker = 3086,
   kFetchBodyStreamOutsideServiceWorker = 3087,
+  kGetComputedStyleOutsideFlatTree = 3088,
 
   // 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/css/css_computed_style_declaration.cc

@@ -307,9 +307,14 @@ bool CSSComputedStyleDeclaration::IsMonospaceFont() const {
 const ComputedStyle* CSSComputedStyleDeclaration::ComputeComputedStyle() const {
   Node* styled_node = this->StyledNode();
   DCHECK(styled_node);
-  return styled_node->EnsureComputedStyle(styled_node->IsPseudoElement()
-                                              ? kPseudoIdNone
-                                              : pseudo_element_specifier_);
+  const ComputedStyle* style = styled_node->EnsureComputedStyle(
+      styled_node->IsPseudoElement() ? kPseudoIdNone
+                                     : pseudo_element_specifier_);
+  if (style && style->IsEnsuredOutsideFlatTree()) {
+    UseCounter::Count(node_->GetDocument(),
+                      WebFeature::kGetComputedStyleOutsideFlatTree);
+  }
+  return style;
 }
 
 Node* CSSComputedStyleDeclaration::StyledNode() const {

tools/metrics/histograms/enums.xml

@@ -25456,6 +25456,7 @@ Called by update_net_error_codes.py.-->
   <int value="3085" label="ModuleDedicatedWorker"/>
   <int value="3086" label="FetchBodyStreamInServiceWorker"/>
   <int value="3087" label="FetchBodyStreamOutsideServiceWorker"/>
+  <int value="3088" label="GetComputedStyleOutsideFlatTree"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">