Move LayoutTestSupport functions into a LayoutTestSupport class

Rather than calling free functions, we should call static functions in a
LayoutTestSupport class so that it's clearer from the call sites where these
functions are defined.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/393943005

git-svn-id: svn://svn.chromium.org/blink/trunk@178211 bbb929c8-8fbe-4397-9dbb-9b2b20218538