Fixing a typo in the MarkerAwareValueProvider name. Also removing additional registration of IValueProviderFactoryProvider from
services.
Add WebRoot node to project.json on MvcSample.Web
[Fixes #1201] Handle virtual paths in FilePathResult
IServiceProvider.GetService now returns null for missing services

Previously GetService would throw if the requested service was missing.
This commit updates MVC to account for the change in behavior.
Replace calls to GetService<T> with GetRequiredService<T>

Even though GetService<T> still exists, GetRequiredService<T> preserves
the old behavior of throwing for missing services.
Split line made over 120 chars by call to GetRequiredService