switch GatherPixelRefs to use SkBaseDevice instead of SkBitmapDevice

BUG=
R=robertphillips@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/70473003

git-svn-id: http://skia.googlecode.com/svn/trunk/src@12259 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed