Add explicit TimeDelta constexpr copy constructor.

MSVC fails to initialize constexpr TimeDelta instances in some
situations.  It turns out (thanks brucedawson@) that including
an explicit constexpr copy constructor works around this.

BUG=635974

Review-Url: https://codereview.chromium.org/2241603002
Cr-Commit-Position: refs/heads/master@{#411805}
2 files changed