Allow optimizing compiler to compute Math.log using untagged doubles.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 5984)
+++ src/heap.h	(working copy)
@@ -2054,8 +2054,9 @@
 
   // Allow access to the caches_ array as an ExternalReference.
   friend class ExternalReference;
-  // Inline implementation of the caching.
+  // Inline implementation of the cache.
   friend class TranscendentalCacheStub;
+  friend class TranscendentalCacheSSE2Stub;
 
   static TranscendentalCache* caches_[kNumberOfCaches];
   Element elements_[kCacheSize];