[CSS Grid Layout] Limit the size of the explicit and implicit grid

Specs allow us to set a limit (maximum size) for both the explicit and implicit
grids so we can protect ourselves from absurdly huge grid specifications. It was
decided to use the recommended limit which is 1 million.

This means that we can remove the old limitation for the number of repetitions
in repeat(). Instead we now clamp the number of repetitions to the maximum
value that allow us to have the maximum number of whole repetitions without
exceeding the track number limit.

BUG=402006

Review URL: https://codereview.chromium.org/613323004

git-svn-id: svn://svn.chromium.org/blink/trunk@183257 bbb929c8-8fbe-4397-9dbb-9b2b20218538