Support 'extend-to-zoom' viewport descriptor value

Based on patch from Kenneth Rohde Christiansen <kenneth.r.christiansen@intel.com>.

See http://dev.w3.org/csswg/css-device-adapt for details.

This new feature is enabled only for UA and user style sheets.

Review URL: https://chromiumcodereview.appspot.com/19632002

git-svn-id: svn://svn.chromium.org/blink/trunk@154511 bbb929c8-8fbe-4397-9dbb-9b2b20218538