Avoid unnecessary descents into nested boxes with floats.

Steal a bit from RenderBlock to avoid descending through the same
stack of nested boxes multiple times while preparing to lay them out.


This CL creates a 4% improvement in the 
Performance/Layout/floats_*_nested.html tests.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170060

Review URL: https://codereview.chromium.org/195363007

git-svn-id: svn://svn.chromium.org/blink/trunk@178193 bbb929c8-8fbe-4397-9dbb-9b2b20218538