DevTools: After clicking on 'Device' dropdown box, the page is 'Emulate' according to selected 'Device' name.

BUG=338616
TBR=dgozman

Review URL: https://codereview.chromium.org/150663010

git-svn-id: svn://svn.chromium.org/blink/trunk@166521 bbb929c8-8fbe-4397-9dbb-9b2b20218538