Add HTTP header X-WAP for MobileFrontend

This will be used to vary varnish cache based on whether or not
we are displaying WAP content. This header will be used for
variance after we finish caching rewrite in MF, that will allow
us to stop varying the cache based on X-Device.

Change-Id: I8534df583ac045e18edb7d856dc08ddd71cb98aa