Make the strict-mode calling convention for contextual calls the default one.

test/mjsunit/regress/regress-2758.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 29 matching lines @@
   for (var j in functions) {
     print(functions[i])
     assertThrows(functions[j], TypeError)
   }
 
   if (i === 3) {
     for (var j in functions)
       %OptimizeFunctionOnNextCall(functions[j]);
   }
 }