Use reference for FillLayer if possible

RenderStyle::backgroundBackgroundLayers() and maskLayers() always
return non-null FillLayer, so we can reference for them.

Also changed accessBackgroundLayers() and accessMaskLayers().

Also changed callers and some callers of callers of the functions.

Review URL: https://codereview.chromium.org/381473002

git-svn-id: svn://svn.chromium.org/blink/trunk@178071 bbb929c8-8fbe-4397-9dbb-9b2b20218538