DevTools: responsive design rulers UI tweaks.
DevTools: responsive design rulers UI tweaks.

BUG=327641
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/335383004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@176328 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs