Remove unused argument in RenderLayer::needsCompositingLayersRebuiltForFilters

The didPaintWithLayers argument is unused. Also added a comment to a FIXME
for future reference.

BUG=343759

Review URL: https://codereview.chromium.org/239533003

git-svn-id: svn://svn.chromium.org/blink/trunk@171705 bbb929c8-8fbe-4397-9dbb-9b2b20218538