Remove the autosizing modes in favor of a dev site link.

Per discussion on http://crbug.com/319092, the autosizing configurations
are going to be confusing to users. This change switches back to just
a single "text autosizing" checkbox that emulates Chrome on Android's
text autosizing behavior. This change also adds a link at the bottom of the
screen emulation page for more information on the developers.google.com site.

A followup patch will add help icons next to some of the checkboxes for
light-weight contextual information.

Preview screenshot: http://pr.gg/devsitelink.png

BUG=319092

Review URL: https://codereview.chromium.org/77723002

git-svn-id: svn://svn.chromium.org/blink/trunk@163062 bbb929c8-8fbe-4397-9dbb-9b2b20218538