Adds counters which track the age of code in the heap during a gc if
--track-gc-object-stats is enabled.

 - Splits RecordObjectStats into RecordObjectStats, RecordCodeSubTypeStats and
   RecordFixedArraySubTypeStats.
 - Renames kNoAge to kNoAgeCodeAge to follow other code age enums and enable
   the name to be used in Macro based initialization of the counters.

BUG=None
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/26179004

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17369 ce2b1a6d-e550-0410-aec6-3dcde31c8c00