Revert change 5989, which causes failures in some benchmarks.

src/math.js

 // Copyright 2006-2008 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 247 matching lines @@
     "max", MathMax,
     "min", MathMin
   ));
 
   // The values here are from the MathFunctionId enum in objects.h.
   %SetMathFunctionId($Math.floor, 1);
   %SetMathFunctionId($Math.round, 2);
   %SetMathFunctionId($Math.abs, 4);
   %SetMathFunctionId($Math.sqrt, 0xd);
   %SetMathFunctionId($Math.pow, 0xe);
-  %SetMathFunctionId($Math.log, 5);
   // TODO(erikcorry): Set the id of the other functions so they can be
   // optimized.
 };
 
 
 SetupMath();