Make window.showModalDialog configurable at runtime

This CL makes it possible to enable and disable showModalDialog with a runtime
flag. Currently, the flag is enabled in stable, but we'll likely change that at
some point soon.

BUG=345831

Review URL: https://codereview.chromium.org/243473002

git-svn-id: svn://svn.chromium.org/blink/trunk@171964 bbb929c8-8fbe-4397-9dbb-9b2b20218538