[DevTools] Replace fontScaleFactor with deviceScaleFactor in device metrics override.

BUG=288959
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/26432005

git-svn-id: svn://svn.chromium.org/blink/trunk@159121 bbb929c8-8fbe-4397-9dbb-9b2b20218538