The SmallestTrackStart method returns a signed int which is never positive, representing the smallest untranslated start among all grid items, clamped by 0 as a maximum.

In practice, though, what we usually want is the index of the first explicit track/line among all tracks/lines, or equivalently, the number of leading implicit tracks/lines.

That number is precisely minus the return value of SmallestTrackStart.
Thus, in grid layout there were various std::abs() to invert the value.

But it's a bit confusing to have an API which returns a non-positive integer, which then needs to be made non-negative.
Therefore, we should rename SmallestTrackStart to ExplicitGridStart, and make it return an unsigned int.

Chromium did this in https://crrev.com/777768