The fields in StackTransferRecipe were public some time ago. Now they
are private, so prepend them with "_".

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ibb94841871fce4c8eca02cb3c369465183bfa5e0
Reviewed-on: https://chromium-review.googlesource.com/877881
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50741}