Add back window.showModalDialog

This is a revert of r167736 plus
changes to make it compile after the
Frame -> LocalFrame rename.

We are temporarily adding back window.showModalDialog
for M35, but plan to attempt removal again in M36.

BUG=345831

Review URL: https://codereview.chromium.org/200713002

git-svn-id: svn://svn.chromium.org/blink/trunk@169277 bbb929c8-8fbe-4397-9dbb-9b2b20218538