Add a use counter and deprecation message for showModalDialog

Mozilla is also interested in deprecating this API. This CL adds a use counter
to see how feasible that might be.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/51033005

git-svn-id: svn://svn.chromium.org/blink/trunk@161186 bbb929c8-8fbe-4397-9dbb-9b2b20218538