We don't need to copy the std::function because we don't modify it's
state.

Bug: v8:8015
Change-Id: I5320f78683fb7c1cca44b73e54312b3cc07b28cd
Reviewed-on: https://chromium-review.googlesource.com/1209782Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Florian Sattler <sattlerf@google.com>
Cr-Commit-Position: refs/heads/master@{#55708}