Removed unused matching behavior.

MatchOnlyUserAgentRules used to be used for an incorrect evaluation of REM
units in media queries. No longer needed.

Review URL: https://codereview.chromium.org/258023004

git-svn-id: svn://svn.chromium.org/blink/trunk@172751 bbb929c8-8fbe-4397-9dbb-9b2b20218538