escape attribute values in test converter that can possibly have regex chars in them

This patch covers a case that was encountered when importing the css shapes test
suite where there were some regex characters in a src attribute. The fix is to
simply escape the attribute values before converting them.

BUG=426468

Review URL: https://codereview.chromium.org/667973004

git-svn-id: svn://svn.chromium.org/blink/trunk@184676 bbb929c8-8fbe-4397-9dbb-9b2b20218538