Use %pure-parser in grammar files to make Bison 3 happy.

This avoids a "deprecated directive, use ‘%pure-parser’ [-Wdeprecated]"
warning in css/CSSGrammar.y and xml/XPathGrammar.y when building with
Bison 3.

Review URL: https://codereview.chromium.org/217423011

git-svn-id: svn://svn.chromium.org/blink/trunk@170525 bbb929c8-8fbe-4397-9dbb-9b2b20218538