GitLab

Selectors with unknown namespace prefixes should be dropped.
Added changes to check if a prefix encountered is unknown,
and if it is so ignoring the selector.

BUG=396945

Review URL: https://codereview.chromium.org/1326403002

git-svn-id: svn://svn.chromium.org/blink/trunk@202015 bbb929c8-8fbe-4397-9dbb-9b2b20218538