Make SkiaCanvas creation call ref() on SkCanvas.

There are two callers of SkiaCanvas::create_canvas(SkCanvas*). In both
cases, we were calling ref() first. It is necessary to call ref() in
both cases, since we have an SkCanvas returned by
SkPictureRecorder::beginRecording, which does not increment the ref
count to account for the caller. (i.e. the SkPictureRecorder has the
only ref until you call ref()).

Rather than leave the ref up to the caller, since the SkiaCanvas does
the unref(), it should also do the ref(). Update it to do so and
document its behavior.

Also, make SkiaCanvas' constructors explicit.

Change-Id: I894d0a71a87587cf8c2b26deb2384dc49ae090ef