Fix reliability crash caused by wrong assert.

src/frame-element.h

Index: src/frame-element.h
===================================================================
--- src/frame-element.h	(revision 3925)
+++ src/frame-element.h	(working copy)
@@ -65,6 +65,9 @@
   }
 
   inline void set_number_info(NumberInfo::Type info) {
+    // Copied elements do not have number info. Instead
+    // we have to inspect their backing element in the frame.
+    ASSERT(!is_copy());
     value_ = value_ & ~NumberInfoField::mask();
     value_ = value_ | NumberInfoField::encode(info);
   }