remove SkRefCnt safeRef() and safeUnref(), and replace the call-sites with

remove SkRefCnt safeRef() and safeUnref(), and replace the call-sites with
SkSafeRef() and SkSafeUnref().

This is basically a bug waiting to happen. An optimizing compiler can remove
checks for null on "this" if it chooses. However, SkRefCnt::safeRef() relies on
precisely this check...

void SkRefCnt::safeRef() {
    if (this) {
        this->ref();
    }
}

Since a compiler might skip the if-clause, it breaks the intention of this
method, hence its removal.

static inline void SkSafeRef(SkRefCnt* obj) {
    if (obj) {
        obj->ref();
    }
}

This form is not ignored by an optimizing compile, so we use it instead.

git-svn-id: http://skia.googlecode.com/svn/trunk@762 2bbb7eff-a529-9590-31e7-b0007b416f81