If you created a project using "Blank App" Windows Store Template before and when you add one of the following:
Visual Studio will automatically add Layout Aware Page classes and styles to your project.
But the problem is,
you'll get a warning; The name "LayoutAwarePage" does not exist in the namespace.
Even though there are techniques which advise you to first Clean the solution and then Build the solution,
my advice to you to solve it is to Rebuild the solution.This will remove Design Time Errors and repackage your application.
After that the problem will be solved and you will be free to use a layout aware page.
Hope that helps!