Stop refcounting SkDeferredCanvas::NotificationClient

User code (i.e. WebKit) is now responsible for the lifetime scope of the notification client.
With http://trac.webkit.org/changeset/125804 skia-side ref counting has become unnecessary and undesirable.
Review URL: https://codereview.appspot.com/6443146

git-svn-id: http://skia.googlecode.com/svn/trunk@5169 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed