Introduce a new AccessorPair type for handling JavaScript accessors.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index e9a43fd4fc7db90730ef96c9818c808663c9e2f6..34311368610d40164821d62d7b76c9c06389dac3 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -69,6 +69,8 @@ class Factory {
   Handle<DeoptimizationOutputData> NewDeoptimizationOutputData(
       int deopt_entry_count,
       PretenureFlag pretenure);
+  // Allocates a pre-tenured empty AccessorPair.
+  Handle<AccessorPair> NewAccessorPair();
 
   Handle<String> LookupSymbol(Vector<const char> str);
   Handle<String> LookupSymbol(Handle<String> str);