[DevTools] Responsive design UI rework.

This change moves most of the emulation under |emulateDevice| setting.

BUG=327641

Review URL: https://codereview.chromium.org/330223004

git-svn-id: svn://svn.chromium.org/blink/trunk@176230 bbb929c8-8fbe-4397-9dbb-9b2b20218538