Avoid a circular ownership dependency between CefBrowserViewImpl and
CefBrowserPlatformDelegate[Chrome]Views (owned by CefBrowserHostBase).
Trigger CefBrowserHostBase destruction when the last CefBrowserViewImpl
reference is released.
This fixes a number of shutdown crashes related to overlay CefBrowsers
still existing at CefShutdown.
To test:
Run `cefclient.exe --use-views --hide-frame --hide-controls`
Add `--enable-chrome-runtime` for the same behavior using the Chrome location
bar instead of a text field.