Floats on clean lines that are dirtied during layout

Floats on clean lines that are dirtied during layout

This is a follow-up fix to https://codereview.chromium.org/1306733003 with some
opportunistic refactoring to remove duplicate code. 

When tying floats to the (current) last line-box in block-flow we may hit a float
in lines previously considered clean but which we have since decided to continue
laying out past (e.g.because one of our floats has changed size). If that happens
we need to add the floats to the linebox rather than bailing early. In
https://codereview.chromium.org/1306733003 we weren't catering for the possibility 
that we may have given up on the plan to stop at clean lines.

In the refactoring I got rid of a superfluous bool and refactored some common code
into a function. Now, if we give up on staying within clean lines we immediately
delete them rather than relying on layout state to figure out if we need to do it
later.


BUG=532712

[rhogan, 2015-09-22 19:41:34]

robhogan@gmail.com changed reviewers:
+ eae@chromium.org

[eae, 2015-09-23 14:48:40]

LGTM

[rhogan, 2015-09-23 16:43:57]

The CQ bit was checked by robhogan@gmail.com

[commit-bot: I haz the power, 2015-09-23 16:44:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1355003002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1355003002/20001

[commit-bot: I haz the power, 2015-09-23 16:53:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-23 16:53:04]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)