Allow externalization of canonical strings also. Only restrict canonical strings

vm/object.h

Index: vm/object.h
===================================================================
--- vm/object.h	(revision 17180)
+++ vm/object.h	(working copy)
@@ -4055,6 +4055,9 @@
   static RawString* ToLowerCase(const String& str,
                                 Heap::Space space = Heap::kNew);
 
+  static bool EqualsIgnoringPrivateKey(const String& str1,
+                                       const String& str2);
+
   static RawString* NewFormatted(const char* format, ...)
       PRINTF_ATTRIBUTE(1, 2);
   static RawString* NewFormattedV(const char* format, va_list args);
@@ -4105,9 +4108,6 @@
   static RawOneByteString* EscapeSpecialCharacters(const String& str,
                                                    bool raw_str);
 
-  static bool EqualsIgnoringPrivateKey(const String& str1,
-                                       const String& str2);
-
   // We use the same maximum elements for all strings.
   static const intptr_t kBytesPerElement = 1;
   static const intptr_t kMaxElements = String::kMaxElements;