[CSS Grid Layout] handle undefined RemainingSpace in computeUsedBreadthOfGridTracks algorithm

The spec defines a different code path for the
computeUsedBreadthOfGridTracks algorithm
http://dev.w3.org/csswg/css-grid/#function-ComputeUsedBreadthOfGridTracks.
The current code has a FIXME that mentions this missing piece in our
implementation.

Basically the track breadth is different when the available size is
undefined and thus, cannot be taken into account during the computations.
The available size is undefined whenever the height is auto or the grid
element has a shrink-to-fit behavior.

It would be also a good idea to rename the function to match the specs so
the function name will start with compute instead of computed.

BUG=331531

Review URL: https://codereview.chromium.org/134323002

git-svn-id: svn://svn.chromium.org/blink/trunk@165692 bbb929c8-8fbe-4397-9dbb-9b2b20218538