Node.js still support older versions of clang, and some of those
versions require us to explicitly declare default constructors for
classes. While updating V8 to 6.7 on Node.js we hit a build failure on
Mac OS X and FreeBSD because there was one constructor not complying
with that rule. This commit fixes it.

R=bmeurer@google.com, franzih@google.com, ofrobots@google.com, yangguo@google.com

Bug: v8:7743
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1f57f0c88c27e4755c9e05f6fedd9def55d8cb77
Reviewed-on: https://chromium-review.googlesource.com/1050666Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#53132}