Make sk_sp operator bool explicit.

The sk_sp class has been using the operator pointer to field as a c++98
version of explicit operator bool. This change updates this class to use
explicit operator bool. The one visible change is that the pointer to
field version isn't quite as explcit, requiring code changes for some
users.

Change-Id: Iddf8fb347b1d3ec33db1af08489c9fd885c9bf08
Reviewed-on: https://skia-review.googlesource.com/130380
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed