Older versions of Clang don't allow using an empty set as the default
parameter of a class with an explicit constructor.
"error: chosen constructor is explicit in copy-initialization"

BUG=chromium:630728

Change-Id: I580073788ce3346d1ecffab336a0fcee210b2e0f
Reviewed-on: https://chromium-review.googlesource.com/431080Reviewed-by: Jim Stichnoth <stichnot@chromium.org>