WebView.GoBack, GoForward, CanGoForward and CanGoBack will be added to the next update.
In the meantime you can use NativeObject:
B4X:
Sub GoBack(wv As WebView)
Dim no As NativeObject = wv
If no.RunMethod("canGoBack", Null).AsBoolean = True Then
no.RunMethod("goBack", Null)
End If
End Sub