Add element_type, swap, operators, fix reset on sk_sp.

include/private/SkTLogic.h

Index: include/private/SkTLogic.h
diff --git a/include/private/SkTLogic.h b/include/private/SkTLogic.h
index a11953705cf1abcb7e4926c502c6e0065b9ff16d..b38fd50435cbccfd5de0dbeff15eca9e0d9fdd88 100644
--- a/include/private/SkTLogic.h
+++ b/include/private/SkTLogic.h
@@ -62,6 +62,8 @@ template <typename T> using add_cv_t = typename std::add_cv<T>::type;
 template <typename T> using add_pointer_t = typename std::add_pointer<T>::type;
 template <typename T> using add_lvalue_reference_t = typename std::add_lvalue_reference<T>::type;
 
+template <typename... T> using common_type_t = typename std::common_type<T...>::type;
+
 template <typename S, typename D,
           bool=std::is_void<S>::value || is_function<D>::value || std::is_array<D>::value>
 struct is_convertible_detector {