DevTools: Migrate totalOffset to getBoundingClientRect to correctly deal with flex based layouts.

R=pfeldman, apavlov

Review URL: https://codereview.chromium.org/52533002

git-svn-id: svn://svn.chromium.org/blink/trunk@160958 bbb929c8-8fbe-4397-9dbb-9b2b20218538