[DevTools] Responsive design mode.

This is an initial responsive design implementation, which includes resizers, grid, rulers and sticky mode (i.e. height is not fixed, but changes with window resize).
Everything is controlled from emulation tab.

BUG=327641

Review URL: https://codereview.chromium.org/258823013

git-svn-id: svn://svn.chromium.org/blink/trunk@174478 bbb929c8-8fbe-4397-9dbb-9b2b20218538