The idea of movable data structures is to pass them by value. This is
also preferred by the style guide.

R=ahaas@chromium.org
CC=sattlerf@google.com

Bug: v8:8015
Change-Id: Ica016425d624f4497e374b25b363c1f2eb49b4c0
Reviewed-on: https://chromium-review.googlesource.com/1245762Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56238}