The relevant spec texts are:
* 8.3. Line-based Placement (https://drafts.csswg.org/css-grid/#grid-placement-int):
  "all implicit grid lines are assumed to have that name
   for the purpose of finding this position."

Only checking this part of the spec it seems pretty clear that Firefox behavior is the good one.

* 9.1. With a Grid Container as Containing Block (https://drafts.csswg.org/css-grid/#abspos-items):
  "If a grid-placement property refers to a non-existent line
   either by explicitly specifying such a line
   or by spanning outside of the existing implicit grid,
   it is instead treated as specifying auto (instead of creating new implicit grid lines)."

However this part makes me wonder we should use "auto" line or the implicit lines in this example.
"referst to a non-existent line", if there are implicit lines that's not possible
as all the implicit lines are assumed to have that name.
For example, if we use "bar" instaed of "foo" for the abspos in the example.
Should we add some clarifications to that sentence?
Filled a CSSWG to ask about this: https://github.com/w3c/csswg-drafts/issues/3445

A note has been added to the spec to clarify the 9.1. section.
So I agree it's a bug on our side.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/80068ae883fa0ba5ae2714c6743191bc49acece9

commit 80068ae883fa0ba5ae2714c6743191bc49acece9
Author: Oriol Brufau <obrufau@igalia.com>
Date: Thu Jan 10 13:00:23 2019

[css-grid] Let abspos items reference implicit grid lines

While they can't create new implicit grid lines, abspos items
can reference existing ones as clarified in
https://github.com/w3c/csswg-drafts/commit/511bb63

This patch makes Blink match Firefox and Edge.

Spec: https://drafts.csswg.org/css-grid/#abspos-items

BUG= 915551 

TEST=external/wpt/css/css-grid/abspos/grid-positioned-items-padding-001.html
TEST=external/wpt/css/css-grid/abspos/grid-positioned-items-unknown-named-grid-line-001.html

Change-Id: Ib363d6cc5c4ec9cc584ebd4a86c33719cea9f54c
Reviewed-on: https://chromium-review.googlesource.com/c/1403656
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Reviewed-by: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#621556}
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/renderer/core/layout/layout_grid.cc
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/renderer/core/layout/layout_grid.h
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/renderer/core/style/grid_positions_resolver.cc
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/renderer/core/style/grid_positions_resolver.h
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/web_tests/external/wpt/css/css-grid/abspos/grid-positioned-items-padding-001.html
[modify] https://crrev.com/80068ae883fa0ba5ae2714c6743191bc49acece9/third_party/blink/web_tests/external/wpt/css/css-grid/abspos/grid-positioned-items-unknown-named-grid-line-001.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/702e9fc024f93d2525d83114516b7cdba6bcc308

commit 702e9fc024f93d2525d83114516b7cdba6bcc308
Author: Oriol Brufau <obrufau@igalia.com>
Date: Thu Jan 10 18:35:43 2019

[css-grid] Remove unused parameters after r621556

The grid_container_style parameter of InitialAndFinalPositionsFromStyle
and SpanSizeForAutoPlacedItem is no longer used after since r621556 and
can be removed.

BUG= 915551 

Change-Id: Iac8baaf86233d9fce6ae70dff8ff5e539e0d18ca
Reviewed-on: https://chromium-review.googlesource.com/c/1405308
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Commit-Queue: Manuel Rego <rego@igalia.com>
Reviewed-by: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#621658}
[modify] https://crrev.com/702e9fc024f93d2525d83114516b7cdba6bcc308/third_party/blink/renderer/core/layout/layout_grid.cc
[modify] https://crrev.com/702e9fc024f93d2525d83114516b7cdba6bcc308/third_party/blink/renderer/core/style/grid_positions_resolver.cc
[modify] https://crrev.com/702e9fc024f93d2525d83114516b7cdba6bcc308/third_party/blink/renderer/core/style/grid_positions_resolver.h