Wpf/WinForms/OffScreen - Standardize InternalIsBrowserInitialized

- Add InternalIsBrowserInitialized to partial class
- Move ThrowExceptionIfDisposed/ThrowExceptionIfBrowserNotInitialized to partial class
- Remove checks from DevToolsExtensions.ExecuteDevToolsMethodAsync as the checks were already in GetBrowser

In WPF it's no longer necessary to have a custom implementation, will leave the Dependency property
(won't action #3033 at this point in time)