Make CSSPropertyName::ToAtomicString() return const-ref.

It calls one out of two functions, and both return a
const AtomicString&, so there's no reason to return an
AtomicString here and take the refcount cost.

Change-Id: Ib8f7d1cd9c29b1752231f8b8ab925e6e801e4321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6357369
Auto-Submit: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432684}
diff --git a/third_party/blink/renderer/core/css/css_property_name.cc b/third_party/blink/renderer/core/css/css_property_name.cc
index 93f199b..30ebcef 100644
--- a/third_party/blink/renderer/core/css/css_property_name.cc
+++ b/third_party/blink/renderer/core/css/css_property_name.cc
@@ -32,7 +32,7 @@
   return custom_property_name_ == other.custom_property_name_;
 }
 
-AtomicString CSSPropertyName::ToAtomicString() const {
+const AtomicString& CSSPropertyName::ToAtomicString() const {
   if (IsCustomProperty()) {
     return custom_property_name_;
   }
diff --git a/third_party/blink/renderer/core/css/css_property_name.h b/third_party/blink/renderer/core/css/css_property_name.h
index ac4d0f5..9c1683a 100644
--- a/third_party/blink/renderer/core/css/css_property_name.h
+++ b/third_party/blink/renderer/core/css/css_property_name.h
@@ -60,7 +60,7 @@
 
   bool IsCustomProperty() const { return Id() == CSSPropertyID::kVariable; }
 
-  AtomicString ToAtomicString() const;
+  const AtomicString& ToAtomicString() const;
 
  private:
   // For HashTraits::EmptyValue().