Fix refcounting in SkiaImageFilterBuilder: change all uses of SkAutoTUnref<SkImageFilter> to RefPtr<SkImageFilter>.

BUG=269884
R=fmalita@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/22507006

git-svn-id: svn://svn.chromium.org/blink/trunk@155840 bbb929c8-8fbe-4397-9dbb-9b2b20218538