cefclient: views: Add ability to pop out the overlay Browser (see #3790)

When running with the overlay Browser enabled (`--show-overlay-browser`),
pressing Alt+O will move the overlay Browser to a new top-level Window.
Pressing Alt+O again or closing the new Window via the close button will
return the Browser to the overlay. Closing the Browser via `window.close()`
(in the new Window or overlay) will dismiss the overlay completely as
required to maintain consistent internal state. Detection of this state is
supported by the new CefBrowserHost::IsReadyToBeClosed method.

Draggable regions in the main Browser are updated to account for the
presence or absence of the overlay Browser. Support for draggable regions
in the overlay Browser in not implemented in cefclient.

Behavior with multiple overlays, `window.close()` and draggable regions
can be tested by adding `--hide-frame --hide-controls`.

libcef/browser/browser_platform_delegate.cc

@@ -63,6 +63,10 @@ class PopupWindowDelegate : public CefWindowDelegate {
     return true;
   }
 
+  cef_runtime_style_t GetWindowRuntimeStyle() override {
+    return browser_view_->GetRuntimeStyle();
+  }
+
  private:
   CefRefPtr<CefBrowserView> browser_view_;