Use constrained values for shouldDisableDesktopWorkarounds.

shouldDisableDesktopWorkarounds() used parsed ViewportArguments instead of the
constrained values, which meant auto and percentage sized @viewport would not
be considered as documents adapted to small screens.

This fix uses the constrained values from PageScaleConstraints instead.

A test for using auto-sizing of @viewport was added.

BUG=280759

Review URL: https://chromiumcodereview.appspot.com/23592015

git-svn-id: svn://svn.chromium.org/blink/trunk@157173 bbb929c8-8fbe-4397-9dbb-9b2b20218538