add SK_SUPPORT_LEGACY_PIXELREF_UNFLATTENABLE guard

this is just so we can stage changing pixelref to not inherit from SkFlattenable

BUG=

Review URL: https://codereview.chromium.org/373803004

git-svn-id: svn://svn.chromium.org/blink/trunk@177613 bbb929c8-8fbe-4397-9dbb-9b2b20218538