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In https://codereview.chromium.org/1132103007 the problem with the trailing float
in fast/block/float/trailing-float.html is that it doesn't have a linebox, so 
when line layout is run on a selection of dirty lineboxes on the parent it is
not re-added to the floating objects list and so doesn't get a paint. So the correct
fix is to ensure any floats not added to the floating objects list during line
layout are added, rather than to attempt to dirty more lines in the hope that
line layout will hit the float, add it to the floats list 
and so ensure it gets a paint.

BUG=510703

Review URL: https://codereview.chromium.org/1298603004

git-svn-id: svn://svn.chromium.org/blink/trunk@200915 bbb929c8-8fbe-4397-9dbb-9b2b20218538