New parser mode for CSSOM @viewport descriptors.

CSSOM value and descriptor parsing for @viewport used parsing rules for normal
CSS property declarations instead, meaning width and height were not parsed as
shorthands.

This is an issue for other CSSOM access to other descriptors too, like for
@font-face.

This fix is minimal for the @viewport case and intended for feedback on the
general approach. I considered not adding new parser modes, but use a bit in
StylePropertySet to indicate a CSSParserMode or a descriptor mode and let two
bits store the mode. With both the current changes and the split, a descriptor
mode would implicitly be strict.

BUG=272718

Review URL: https://chromiumcodereview.appspot.com/22917005

git-svn-id: svn://svn.chromium.org/blink/trunk@157955 bbb929c8-8fbe-4397-9dbb-9b2b20218538