Fix magic constants for max count of arguments in webkit-filter

Parsing webkit-filter in function parseFilter(),
we set var maximumArgumentCount.
However this var wasn't used,
instead we used hardcoded magic numbers
in numerous checks for filters.
I deleted this checks and wrote one check with
var maximumArgumentCount.

R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/473173002

git-svn-id: svn://svn.chromium.org/blink/trunk@180565 bbb929c8-8fbe-4397-9dbb-9b2b20218538