MakeLayout Test editing/selection/fake-drag.html stable

This patch changes LayoutTest editing/selection/fake-drag.html stable by starting test steps after document loading, invoking the first step, step0(), as load event handler, rather than middle of document loading.

BUG=263991
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/20220003

git-svn-id: svn://svn.chromium.org/blink/trunk@154905 bbb929c8-8fbe-4397-9dbb-9b2b20218538