DevTools: [Elements] Center focused tree element vertically when selecting inspected node in page

In order to avoid visual jitter, if an element is already visible in the tree, the scroll will not occur.

R=aandrey@chromium.org, pfeldman, vsevik
BUG=305562

Review URL: https://codereview.chromium.org/27002003

git-svn-id: svn://svn.chromium.org/blink/trunk@159447 bbb929c8-8fbe-4397-9dbb-9b2b20218538