DevTools: Redesign grid lines layout on Timeline grid.

Grid spans are made round values. Grid lines are placed at positions
that are multiples of a chosen span.

BUG=250919
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/14137021

git-svn-id: svn://svn.chromium.org/blink/trunk@152768 bbb929c8-8fbe-4397-9dbb-9b2b20218538