Revert 161813 "Fix error recovery in CSS source data extraction."

BUG=318678

> 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

TBR=serya@chromium.org

Review URL: https://codereview.chromium.org/71523002

git-svn-id: svn://svn.chromium.org/blink/trunk@161917 bbb929c8-8fbe-4397-9dbb-9b2b20218538