Unnest RenderLayerCompositor's update recursion

Previously, RenderLayerCompositor::updateCompositingLayers called itself
recursively. This CL factors out the recursive walk into a separate function.
I've also cleaned up some related code to have stricter ASSERTs instead of
early exits.

This CL also removes fast/repaint/event-while-loading.html, which is mostly
testing the test harness rather than the shipping code. I've also rebaselined
compositing/repaint/fixed-pos-with-abs-pos-child-scroll.html now that
startTrackingRepaints triggers a layout.

Review URL: https://codereview.chromium.org/227893003

git-svn-id: svn://svn.chromium.org/blink/trunk@171009 bbb929c8-8fbe-4397-9dbb-9b2b20218538