Refactor the viewport code to be more understandable

Introduce a shouldOverrideLegacyViewport method and use it instead
of comparing types manually.

Make the setViewportArguments method handle both legacy and @viewport,
instead of just the latter.

Now the code is a bit more self documenting, so reword some of the
comments.

Review URL: https://chromiumcodereview.appspot.com/23809007

git-svn-id: svn://svn.chromium.org/blink/trunk@157366 bbb929c8-8fbe-4397-9dbb-9b2b20218538