Work around empty set default parameter compilation issue. 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/431080 Reviewed-by: Jim Stichnoth <[email protected]>