Closes the current in-app web view, if one was previously opened by
launchUrl.
This works only if supportsCloseForLaunchMode returns true for the mode
that was used by launchUrl.
Returns true if mode is supported by the current platform implementation.
Calling launchUrl with an unsupported mode will fall back to a supported
mode, so calling this method is only necessary for cases where the caller
needs to know which mode will be used.
Returns true if closeInAppWebView is supported for mode in the current
platform implementation.
If this returns false, closeInAppWebView will not work when launching
URLs with mode.