It is expected that temporarily used strings die while they are
in new heap.  So we can avoid to pay a heavy cost to externalize
them.  If they are used for times, externalization will happen
when they move to an old heap.

BUG=chrmoium:606093

Review-Url: https://codereview.chromium.org/2046933002
Cr-Commit-Position: refs/heads/master@{#36907}