Fix error recovery in CSS source data extraction.

An invalid selector of the last rule inside a @media rule would prevent correct source data extraction.

BUG=317499

Review URL: https://codereview.chromium.org/68373004

git-svn-id: svn://svn.chromium.org/blink/trunk@161813 bbb929c8-8fbe-4397-9dbb-9b2b20218538