[css3-text] Minor fixes on CSSParser for text-decoration

Fixed an issue that prevents an invalid access for CSS parsed values.
This fix also prepares the terrain for the upcoming text-decoration
shorthand implementation.

BUG=165462
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/20093008

git-svn-id: svn://svn.chromium.org/blink/trunk@154847 bbb929c8-8fbe-4397-9dbb-9b2b20218538