[GridNG] Fix failing scrollable overflow tests.

There were never passing in legacy, but is relatively trivial to pass
in NG.

Effectively NG has the concept of the "inflow-bounds" which for grid
layout is the size of the grid. Legacy never has this clear separation
(kinda using the intrinsic-block-size as a proxy).

Additionally fixes the tests (these have never passed in any browser
so had some "obvious" mistakes).

Bug: 1045599
Change-Id: I990f450ee7ec00a2fd730d402e25632b8c748cd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2727860
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#859503}