Don't externalize fresh strings.

Don't externalize fresh strings.

With the current API the embedder has to extrenalize a string each
time a string is encountered to avoid the cost of repeated character
copying/conversion. The issue here is that the externalization cost
itself is non-negligible (both in time and space) and should not be
paid for a rarely used string. This change is an attempt to predict a
string's usage frequency based on its freshness. A string is
considered fresh if it was recently allocated in the new space.

Review URL: http://codereview.chromium.org/608006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3878 ce2b1a6d-e550-0410-aec6-3dcde31c8c00