not seem to impact performance and makes it a lot easier for people
embedding V8 to run their application under valgrind.

If we observe a negative performance impact, we will need to revise
this again.

Review URL: http://codereview.chromium.org/13612.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2662 ce2b1a6d-e550-0410-aec6-3dcde31c8c00