Make SkSpan C++11 compatible.

In C++11 constexpr operators cannot be defaulted.

Change-Id: Ibc73d9b9291f3e1a45bb5c0a7ff59c2b90a3e380
Reviewed-on: https://skia-review.googlesource.com/c/191567
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>