[CSS Grid Layout] Store m_grid across layout

This patch implements the logic to correctly invalidate m_grid
whenever needed. This will make it possible to speed up painting
across layout as we can use it to select the right sub-grid to be
painted.

The added tests are the blind spots in our testing (we already have
test checking grid items removal / addition, ...).

BUG=248151

Review URL: https://chromiumcodereview.appspot.com/21205004

git-svn-id: svn://svn.chromium.org/blink/trunk@155262 bbb929c8-8fbe-4397-9dbb-9b2b20218538