Add XSSInfo::isSafeToSendToAnotherThread() and call it in debug builds.

Follow-on to https://codereview.chromium.org/24816003/, adds thread-safe
checks for the XSSInfo class. Also refactors a free-standing function into a
XSSInfo method as additonal cleanup, re-arranging order of methods to match the 
header file.

BUG=299132
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/25014005

git-svn-id: svn://svn.chromium.org/blink/trunk@158477 bbb929c8-8fbe-4397-9dbb-9b2b20218538