DevTools: [MQI] Highlight media query markers on serif click

This patch runs CSS animation to highlight media query markers whenever
a click on serif happens.

R=dgozman@chromium.org, dgozman, pfeldman

Review URL: https://codereview.chromium.org/345373004

git-svn-id: svn://svn.chromium.org/blink/trunk@176844 bbb929c8-8fbe-4397-9dbb-9b2b20218538