Fix style erros in XPath-related files.

This is a preparation to improve WebCore::XPath::EvaluationContext.
This CL should have no behavior changes.

BUG=

Review URL: https://codereview.chromium.org/344553002

git-svn-id: svn://svn.chromium.org/blink/trunk@176388 bbb929c8-8fbe-4397-9dbb-9b2b20218538