Move DiscardablePixelRef.cpp/.h to platform/graphics/

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26433004

git-svn-id: svn://svn.chromium.org/blink/trunk@159800 bbb929c8-8fbe-4397-9dbb-9b2b20218538