Multicol: Add test that re-lays out a float and pushes it downwards at the same time.

Multicol: Add test that re-lays out a float and pushes it downwards at the same time.

The test passes, but the code seems somewhat brittle, so better make sure that
it keeps working.

Inline-level layout of floats is weird. LayoutBlockFlow::layoutInlineChildren()
lays out the float before its position is updated, and then marks the lines that
it thinks are affected as dirty (it may find the wrong lines here, but I haven't
been able to make anything fail because of it). For pagination this
additionally means that the pagination strut may be wrong (since the float's
position hasn't been updated). Luckily, we get an opportunity to lay out again
in insertFloatingObject() (still at the old position, though), and yet another
opportunity in positionNewFloats() (this time at the updated position).

R=jchaffraix@chromium.org,leviw@chromium.org,robhogan@gmail.com

Committed: https://crrev.com/011ad4bb05558061cb4c8caf0204b552c3f9d77d
Cr-Commit-Position: refs/heads/master@{#352587}

[leviw_travelin_and_unemployed, 2015-10-06 10:54:03]

lgtm

https://codereview.chromium.org/1375913003/diff/1/third_party/WebKit/LayoutTe...
File third_party/WebKit/LayoutTests/fast/multicol/relayout-and-push-float.html
(right):

https://codereview.chromium.org/1375913003/diff/1/third_party/WebKit/LayoutTe...
third_party/WebKit/LayoutTests/fast/multicol/relayout-and-push-float.html:11:
<br>
Are both of these br's necessary?

[mstensho (USE GERRIT), 2015-10-06 11:31:11]

https://codereview.chromium.org/1375913003/diff/1/third_party/WebKit/LayoutTe...
File third_party/WebKit/LayoutTests/fast/multicol/relayout-and-push-float.html
(right):

https://codereview.chromium.org/1375913003/diff/1/third_party/WebKit/LayoutTe...
third_party/WebKit/LayoutTests/fast/multicol/relayout-and-push-float.html:11:
<br>
On 2015/10/06 10:54:02, leviw wrote:
> Are both of these br's necessary?

One is actually enough. Fixed.

[mstensho (USE GERRIT), 2015-10-06 11:31:22]

The CQ bit was checked by mstensho@opera.com

[mstensho (USE GERRIT), 2015-10-06 11:31:23]

The patchset sent to the CQ was uploaded after l-g-t-m from leviw@chromium.org
Link to the patchset: https://codereview.chromium.org/1375913003/#ps20001
(title: "Code review. Now 20% more BR-free.")

[commit-bot: I haz the power, 2015-10-06 11:31:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1375913003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1375913003/20001

[commit-bot: I haz the power, 2015-10-06 12:24:43]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-06 12:26:27]

Patchset 2 (id:??) landed as
https://crrev.com/011ad4bb05558061cb4c8caf0204b552c3f9d77d
Cr-Commit-Position: refs/heads/master@{#352587}