Add comment to compilation-cache.cc explaining choice of 5 generations.

src/compilation-cache.cc

Index: src/compilation-cache.cc
===================================================================
--- src/compilation-cache.cc	(revision 3056)
+++ src/compilation-cache.cc	(working copy)
@@ -43,6 +43,8 @@
 static const int kEvalContextualGenerations = 1;
 static const int kRegExpGenerations = 1;
 #else
+// The number of ScriptGenerations is carefully chosen based on histograms.
+// See issue 458: http://code.google.com/p/v8/issues/detail?id=458
 static const int kScriptGenerations = 5;
 static const int kEvalGlobalGenerations = 2;
 static const int kEvalContextualGenerations = 2;