DevTools: Introduce suggest box overlay for faster suggest box layout.

R=pfeldman, lushnikov, apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/257563002

git-svn-id: svn://svn.chromium.org/blink/trunk@172480 bbb929c8-8fbe-4397-9dbb-9b2b20218538