[DevTools] Implement fitWindow for responsive design.

This patch adds offset and scale parameters to protocol method
Page.setDeviceMetricsOverride. Those are applied when fitWindow
mode is off.

In responsive design mode, when checkbox "fit" is on, page
is scaled down to fit space. When dragging to resize, scale
doesn't change until user releases the drag handle.

BUG=327641

Review URL: https://codereview.chromium.org/333593005

git-svn-id: svn://svn.chromium.org/blink/trunk@176650 bbb929c8-8fbe-4397-9dbb-9b2b20218538