DevTools: Avoid scrolling suggest box selected item into view when possible.

scrollIntoView forces extra layout and should be avoided when selectedIndex is trivial.

R=pfeldman,lushnikov,apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/250563003

git-svn-id: svn://svn.chromium.org/blink/trunk@172487 bbb929c8-8fbe-4397-9dbb-9b2b20218538