Refactor UnionOfKeys into ElementsAccessor

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 8f9f18c776e218b941c66888e47d0af59319cabe..ff13aedcce6f9ba667cccbcdc20b8fd49f4c3f2e 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2128,10 +2128,6 @@ class FixedArray: public FixedArrayBase {
   // Compute the union of this and other.
   MUST_USE_RESULT MaybeObject* UnionOfKeys(FixedArray* other);
 
-  // Compute the union of this and other.
-  MUST_USE_RESULT MaybeObject* UnionOfDoubleKeys(
-      FixedDoubleArray* other);
-
   // Copy a sub array from the receiver to dest.
   void CopyTo(int pos, FixedArray* dest, int dest_pos, int len);