Correct fragmentainer offset when considering break before grid item.
The fragmentation utils (including MovePastBreakpoint() and
PropagateSpaceShortage()) generally need an offset relatively to the
start of the fragmentainer, not the start of the parent container. Grid
fragmentation did the latter, which, if the grid container itself wasn't
at the start of the fragmentainer, we could incorrectly believe that we
had more space available than what we actually had, so that a grid item
would be allowed to stay in the current fragmentainer (instead of
breaking before it), even if there wasn't enough room for it. We'd then
later on detect that the grid container fragment itself was too large to
fit, and therefore break before the grid container.
Bug: 353880548
Change-Id: Id5f7a0f88ec107d048a8a1be276c1fce7b24dc1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5721349
Reviewed-by: Ian Kilpatrick ikilpatrick@chromium.org
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Cr-Commit-Position: refs/heads/main@{#1329610}