Fix failure in DrtFixedEdit

[ojhad]

DrtFixedEdit is failing because we are currently trying to read the scheme of a relative uri here:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedPage.cs

Line 618 in 3a42862

(inputUri.Scheme == PackUriHelper.UriSchemePack && !String.IsNullOrEmpty(inputUri.Fragment)))

This failure was introduced due to changes that were made to the above if statement here:
https://github.com/dotnet/wpf/pull/969/files#diff-d6cd62f214719773a7a26ab93a0235b8

The failure was introduced by removing the CanNavigateToUri method, which was blocking the execution of the following inputUri.Scheme statement for relative URIs.