Fixing one more conflict in CSS grammar.
Fixing one more conflict in CSS grammar.

Grammar allows to reduce CSS like '@charset "xxx"; ...' to both maybe_charset and ignored_charset. Adding error into ignored charset resolves this conflict.

BUG=

Review URL: https://chromiumcodereview.appspot.com/16174006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152035 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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