Revert 252883 "Remove tests and code for showModalDialog"

This CL broke show-modal-dialog tests in Blink.
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=fast/dom/Window/open-window-min-size.html,fast/events/scroll-event-during-modal-dialog.html,fast/events/show-modal-dialog-onblur-onfocus.html,fast/harness/show-modal-dialog.html

> Remove tests and code for showModalDialog
> 
> This deletes the chromium-side implementation logic and a few tests for
> showModalDialog. I believe more simplification is possible to do without the
> design constraints imposed by showModalDialog but we should probably start
> small in case we run into unexpected compat issues.
> 
> R=darin
> BUG=345831
> 
> Review URL: https://codereview.chromium.org/165793003

TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/177263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252889 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252889