diff --git a/layout/style/nsCSSValue.cpp b/layout/style/nsCSSValue.cpp
index 5f30efbc529a..a020a68473ca 100644
--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -2775,16 +2775,6 @@ nsCSSValuePairList_heap::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf
   return n;
 }
 
-size_t
-nsCSSValue::Array::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
-{
-  size_t n = aMallocSizeOf(this);
-  for (size_t i = 0; i < mCount; i++) {
-    n += mArray[i].SizeOfExcludingThis(aMallocSizeOf);
-  }
-  return n;
-}
-
 css::URLValueData::URLValueData(already_AddRefed<PtrHolder<nsIURI>> aURI,
                                 nsStringBuffer* aString,
                                 already_AddRefed<PtrHolder<nsIURI>> aBaseURI,
diff --git a/layout/style/nsCSSValue.h b/layout/style/nsCSSValue.h
index 1da9e65606ab..ea7c2b35bc0a 100644
--- a/layout/style/nsCSSValue.h
+++ b/layout/style/nsCSSValue.h
@@ -1117,7 +1117,14 @@ private:
     }
   }
 
-  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+  {
+    size_t n = aMallocSizeOf(this);
+    for (size_t i = 0; i < mCount; i++) {
+      n += mArray[i].SizeOfExcludingThis(aMallocSizeOf);
+    }
+    return n;
+  }
 
 private:
   Array(const Array& aOther) = delete;