Pass in ScriptState to showModalDialog

This CL passes ScriptState to DialogHandler and makes sure that the asynchronous callbacks of DialogHandler uses a correct context.

This CL is potentially changing behavior (i.e., correcting the context used by the DialogHandler), but it's not easy to test it because there is no way to run modal things in internals.executeInIsolatedWorld(). (Also note that the modal dialog is going to be deprecated.)

BUG=357144

Review URL: https://codereview.chromium.org/297803004

git-svn-id: svn://svn.chromium.org/blink/trunk@174635 bbb929c8-8fbe-4397-9dbb-9b2b20218538