[CSS Grid Layout] Fix the handling of infinity in track growth limits

The growth limit of content sized tracks is initialized to
infinity which is internally represented as -1. We were not
specialcasing this situation, and thus, -1 was used in the
computations as any other value. This change makes the code aware
of the existence of infinites (like when sorting tracks by groth
potential or when initializing the track growth limits).

There was another bug related to infinities. The code that was
replacing a infinite growth limit by a finite one was not using
the proper indexes so the tracks that were being updated were the
wrong ones.

Finally this change unveiled a bug in the call to
distributeSpaceToTracks(). Sometimes the space to be distributed
might be negative or 0. In the former case that lead to wrong
computations while in the latter we were doing some extra job by
calling the function as there were nothing to do.

BUG=392200

Review URL: https://codereview.chromium.org/410163002

git-svn-id: svn://svn.chromium.org/blink/trunk@179621 bbb929c8-8fbe-4397-9dbb-9b2b20218538