Recompute percentage based @viewport on resize.

Percentage values were resolved to px lengths at cascade time for @viewport
widths and heights. This change keeps them as percentage values and recompute
the actual viewport from the initial viewport and the percentage value when
the intial viewport changes (window resize). Likewise for viewport relative
units (vw, vh, vmin, vmax).

This fixes constrain-006 and constrain-007 in the w3c testsuite.

This CL also disallows 'initial' and 'inherit' as descriptor values and returns 'auto' for unknown non-primitive values to avoid crashers and improve on the original CL for the same bug: https://codereview.chromium.org/22549002/

BUG=269132

Review URL: https://chromiumcodereview.appspot.com/23604037

git-svn-id: svn://svn.chromium.org/blink/trunk@157225 bbb929c8-8fbe-4397-9dbb-9b2b20218538