DevTools: Simplify suggest box styles and move them to one file.

R=lushnikov,pfeldman,apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/257603002

git-svn-id: svn://svn.chromium.org/blink/trunk@172490 bbb929c8-8fbe-4397-9dbb-9b2b20218538