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.

lgtm'd pre-merge at https://codereview.chromium.org/1355003002

TBR=eae@chromium.org

BUG=532712
Committed: https://crrev.com/1461e48329b74a0bf26496cd581096af0e9b7380
Cr-Commit-Position: refs/heads/master@{#351010}

[rhogan, 2015-09-27 18:12:46]

The CQ bit was checked by robhogan@gmail.com

[commit-bot: I haz the power, 2015-09-27 18:12:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1367363002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1367363002/1

[commit-bot: I haz the power, 2015-09-27 19:20:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-27 19:21:27]

Patchset 1 (id:??) landed as
https://crrev.com/1461e48329b74a0bf26496cd581096af0e9b7380
Cr-Commit-Position: refs/heads/master@{#351010}