Print AST needs access to the internalized strings, so make sure AST
is internalized before we print it.

BUG=v8:5203

Change-Id: Ia4995147feb7ec466523a0c4a89620749b23dcab
Reviewed-on: https://chromium-review.googlesource.com/593648Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47006}