Chrome Version       : 56.0.2914.3
OS Version: OS X 10.12.1


Window-Modal sheets probably don't fit into the proposed tab-closable modality behaviours.

However, they're not that easy to get rid of.

One issue is window resize of the dialog. Resizing a sheet resizes it *and* repositions it. We need to hook into the mouse events somehow. It's not feasible just to observe for a window resize in the normal way because repositioning it will move the window under the mouse and immediately want the window to be resized again.

A fix for this might be to simply disable the ability to resize dialogs, the way it's done on other views platforms..

The rest is fairly straightforward.
 - WebContentsModalDialogHostCocoa needs to be fleshed out - it's missing a lot of functionality
  - alternatively, auto-convert everything to WebModal and "assume" we can always get a WebContents (or have a graceful fallback)
 - CreateBrowserModalDialogViews() should opt in to 'requires_positioning`
 - a bit of refactoring fallout.

WIP in https://codereview.chromium.org/2551773002 gets most of the way there.