When using `views::Widget` to create window, Chromium will automatically
use a `BridgedContentView` as content view, which however does not
support draggable regions inside it.
By fallback to `FullSizeContentView` we can work around this problem,
with the price of losing the ability to use `views::View` APIs. Since
we don't expect users to use the new `View` APIs in `BrowserWindow`
anyway, it should not be a problem.
This change does not affect users of `TopLevelWindow`, and for users of
`BrowserWindow` there is nothing to lose.
In the long term we should look into how to make draggable regions work
with `BridgedContentView`. The related Chromium code is still being
changed rapidly, we can wait until Chromium migrated its
`NativeAppWindowCocoa` class to use `views::Widget`.
