DevTools: check for canForceCompositingMode before trying to emulate device metrics.

BUG=288959,308422

Review URL: https://codereview.chromium.org/50043007

git-svn-id: svn://svn.chromium.org/blink/trunk@161082 bbb929c8-8fbe-4397-9dbb-9b2b20218538