Don't create a linebox for trailing floats unless it's required

We should only create a linebox for trailing floats if we are going
to add some floats to it and we will only have floats to add if
they appeared after a linebreak. 

If there was no linebreak then our superfluous linebox won't get 
any floats but it will still push down the baseline for its 
renderer unnecessarily - confusing other elements on the line.

So to avoid this - only create the linebox if we have a float to
add to it.

BUG=133619

Review URL: https://codereview.chromium.org/505803003

git-svn-id: svn://svn.chromium.org/blink/trunk@181290 bbb929c8-8fbe-4397-9dbb-9b2b20218538