Read the default min-width value from the UA style sheets

When enabling the viewport meta tag the default min layout width is hardcoded
to 980. Now we get that value from the UA style sheets instead making it
extend-to-zoom on desktop when enabled.

The former setting has been removed.

BUG=358687
NOTRY=true

Review URL: https://codereview.chromium.org/219123002

git-svn-id: svn://svn.chromium.org/blink/trunk@170655 bbb929c8-8fbe-4397-9dbb-9b2b20218538