Use SkXfermode::Mode enum instead of RefPtr<SkXfermode>

PassRefPtr<SkXfermode> requires difficult memory management between skia and
blink. In addition, this CL optimizes SkXfermode::Mode selecting code.

BUG=407079

Review URL: https://codereview.chromium.org/506613002

git-svn-id: svn://svn.chromium.org/blink/trunk@180860 bbb929c8-8fbe-4397-9dbb-9b2b20218538