DevTools: revealer should bring frontend host to front

This patch changes revealer to bring frontend host to front.
This is important for Media Query Inspector revealing functionality when
devtools are in undocked state.

R=dgozman, pfeldman

Review URL: https://codereview.chromium.org/350553003

git-svn-id: svn://svn.chromium.org/blink/trunk@176980 bbb929c8-8fbe-4397-9dbb-9b2b20218538