DevTools: [Styles] Fix color picker following the scrolled swatch

This regressed in r169860.

R=dgozman, vsevik
BUG=392401
NOTRY=true

Review URL: https://codereview.chromium.org/376133002

git-svn-id: svn://svn.chromium.org/blink/trunk@177746 bbb929c8-8fbe-4397-9dbb-9b2b20218538