Remove showModalDialog layout tests

This removes most of the automatic and manual tests using
showModalDialog.  The remaining tests are window property tests and
tests to make sure you can't call it from an iframe that can get cleaned
up when the function gets removed once and for all.

R=abarth@chromium.org,jamesr@chromium.org
BUG=345831

Review URL: https://codereview.chromium.org/512533002

git-svn-id: svn://svn.chromium.org/blink/trunk@181064 bbb929c8-8fbe-4397-9dbb-9b2b20218538