fix chicken/egg issue with fixed backgrounds

If the background becomes fixed in style recalc, set
a flag in RenderLayerCompositor and handle it after
compositing layer update.

BUG=343132

Review URL: https://codereview.chromium.org/232413002

git-svn-id: svn://svn.chromium.org/blink/trunk@171227 bbb929c8-8fbe-4397-9dbb-9b2b20218538