Enable string pooling in the VS release build.  This saves 46k for a v8_shell_sample.exe, I didn't measure performance or the overall change to a Chromium build.  I think this option should always make sense anyway, unless we're using some string constants incorrectly.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@365 ce2b1a6d-e550-0410-aec6-3dcde31c8c00