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

Review URL: http://codereview.chromium.org/267092

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3065 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/compilation-cache.cc

@@ -43,6 +43,8 @@ static const int kEvalGlobalGenerations = 1;
 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;