[css-grid] Fix positioned items in grids with implicit tracks

[css-grid] Fix positioned items in grids with implicit tracks

We were detecting wrongly the first and last explicit grid lines
during the layout of positioned grid items.
We have to consider that the grid can have implicit tracks created
previously by regular grid items.

BUG=273898
TEST=fast/css-grid-layout/grid-positioned-items-within-grid-implicit-track.html

Committed: https://crrev.com/63585e1ac3b452938a34df46855342d90005ed46
Cr-Commit-Position: refs/heads/master@{#370375}

[Manuel Rego, 2016-01-19 18:14:22]

rego@igalia.com changed reviewers:
+ cbiesinger@chromium.org, jfernandez@igalia.com, svillar@igalia.com

[Manuel Rego, 2016-01-19 18:14:23]

Simple fix for positioned items.

[svillar, 2016-01-20 10:10:28]

lgtm

[Manuel Rego, 2016-01-20 10:18:53]

The CQ bit was checked by rego@igalia.com

[commit-bot: I haz the power, 2016-01-20 10:19:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1608943003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1608943003/20001

[commit-bot: I haz the power, 2016-01-20 13:22:01]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-01-20 13:22:50]

Description was changed from

==========
[css-grid] Fix positioned items in grids with implicit tracks

We were detecting wrongly the first and last explicit grid lines
during the layout of positioned grid items.
We have to consider that the grid can have implicit tracks created
previously by regular grid items.

BUG=273898
TEST=fast/css-grid-layout/grid-positioned-items-within-grid-implicit-track.html
==========

to

==========
[css-grid] Fix positioned items in grids with implicit tracks

We were detecting wrongly the first and last explicit grid lines
during the layout of positioned grid items.
We have to consider that the grid can have implicit tracks created
previously by regular grid items.

BUG=273898
TEST=fast/css-grid-layout/grid-positioned-items-within-grid-implicit-track.html

Committed: https://crrev.com/63585e1ac3b452938a34df46855342d90005ed46
Cr-Commit-Position: refs/heads/master@{#370375}
==========

[commit-bot: I haz the power, 2016-01-20 13:22:51]

Patchset 2 (id:??) landed as
https://crrev.com/63585e1ac3b452938a34df46855342d90005ed46
Cr-Commit-Position: refs/heads/master@{#370375}