DevTools: [Responsive design] shift-drag resize widget for more precise dragging

This patch makes it possible to shift-drag resize widgets on responsive
design page to slow down their movement.

BUG=380190

Review URL: https://codereview.chromium.org/312813002

git-svn-id: svn://svn.chromium.org/blink/trunk@175473 bbb929c8-8fbe-4397-9dbb-9b2b20218538