[CSS Grid Layout] Support calc() breadth track size

The RenderGrid support for calc() was already there but we had to add also the CSSComputedStyle support. According to the specs, http://dev.w3.org/csswg/css-grid/#resolved-track-list, we must show the used value. This change also brings several new tests.

BUG=297689

Review URL: https://codereview.chromium.org/23472008

git-svn-id: svn://svn.chromium.org/blink/trunk@161787 bbb929c8-8fbe-4397-9dbb-9b2b20218538