[CSSMQ] Use initial values as base for relative units.

Don't use computed style for root element as base for relative lengths. The
media queries spec says:

"Relative units in media queries are based on the initial value, which means
that units are never based on results of declarations."

BUG=269322

Review URL: https://chromiumcodereview.appspot.com/22258007

git-svn-id: svn://svn.chromium.org/blink/trunk@155692 bbb929c8-8fbe-4397-9dbb-9b2b20218538