[DevTools] Switch from DIP to CSS pixels in device mode.

Rulers, grid and media queries are now rendered in CSS pixels.
Additionally, a content size rectangle is drawn for visual feedback.

BUG=none

Review URL: https://codereview.chromium.org/351303003

git-svn-id: svn://svn.chromium.org/blink/trunk@177098 bbb929c8-8fbe-4397-9dbb-9b2b20218538