Change (correct) SkImageRef's ownership rule for streams to the skia standard

Change (correct) SkImageRef's ownership rule for streams to the skia standard
pattern: the receiver will call ref() if it chooses when passed to a constructor
or setter, so the caller must balance its ownership itself. This matches how all
other refcnt objects are handled.

git-svn-id: http://skia.googlecode.com/svn/trunk@120 2bbb7eff-a529-9590-31e7-b0007b416f81