[CSS Grid Layout] ASSERTION FAILED: growthShare != infinity

The ASSERT which was added in r179621 as part of the fix for the infinities
handling is not totally correct due to the way we internally represent the
infinity value (as the integer -1). The assertion is partially correct as it
ensures that growthShare is never infinity but the problem is that -1 is valid
value for growthShare (if track.usedBreadth = track.maxBreadth + 1).

The ASSERT was fine tuned so that it accepts the -1 value for growthShare
whenever we know it does not mean infinity.

BUG=410275

Review URL: https://codereview.chromium.org/642443003

git-svn-id: svn://svn.chromium.org/blink/trunk@184064 bbb929c8-8fbe-4397-9dbb-9b2b20218538