The 'element_type' typedef is to play nice with std::pointer_traits.
The full complement of operators and swap to match unique_ptr so that
sk_sp can be properly compared to nullptr and used with standard
containers.
Update to 'reset' so that calling 'unref' is the last operation.
This also adds tests for these changes, and sets the fPtr to nullptr
in debug for easier bug finding.
Review URL: https://codereview.chromium.org/1773453002
c901c11549