Add a runtime flag for XSLT

This CL adds a runtime flag for XSLT so we can experiment with disabling it.

R=eseidel

Review URL: https://codereview.chromium.org/25279004

git-svn-id: svn://svn.chromium.org/blink/trunk@158574 bbb929c8-8fbe-4397-9dbb-9b2b20218538