Move LayoutTestHelper files to testing/helper.

This is another step into removing the Tools/DumpRenderTree/ directory.

BUG=None
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22105002

git-svn-id: svn://svn.chromium.org/blink/trunk@155498 bbb929c8-8fbe-4397-9dbb-9b2b20218538