[DevTools] Blink-side APIs for device metrics emulation in content.

BUG=288959
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/24272003

git-svn-id: svn://svn.chromium.org/blink/trunk@158662 bbb929c8-8fbe-4397-9dbb-9b2b20218538