DevTools: [MQI] add tooltips for ruler serifs

The patch adds tooltips with actual pixel values of serifs, representing
CSS breakpoints on CSS ruler.

BUG=391240
R=pfeldman, dgozman

Review URL: https://codereview.chromium.org/478953002

git-svn-id: svn://svn.chromium.org/blink/trunk@180411 bbb929c8-8fbe-4397-9dbb-9b2b20218538