According to the style guide, we pass unique_ptr by value, or by
reference in rare cases, but never by rvalue reference.

R=ahaas@chromium.org

Bug: v8:7109
Change-Id: I6cfdb8b61fada532ce334e2da419598085a2b408
Reviewed-on: https://chromium-review.googlesource.com/803716Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49820}