[css-device-adapt] Implemented spec changes.

Modified implementation to match spec changes:

- Allow zero lengths for initial and actual viewport sizes.
- auto width and height lengths depending on aspect-ratio, never on zoom.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/16826008

git-svn-id: svn://svn.chromium.org/blink/trunk@154237 bbb929c8-8fbe-4397-9dbb-9b2b20218538