Removing libwebrtc dependency from Nix

A Platform API for WebRTC will be added in the future and this dependency must rely on the client app.
Removing BUILDING_NIX__ dependencies on places that it sould not have

TestWebKitAPI and MiniBrowser were depending on BUILDING_NIX__ and they should not, since they are supposed to be client
applications.
Removing PrivatePtr

We do not need that in our Platform API. We can take advantage of RefPtr.
Adding macros that inject the common code that all Platform API code will have

DEFINE_BASE_NIX_API_INTERNAL -> Add all copy constructors and assignment operators to convert to/from WebCore <->
NixPlatform types

DEFINE_NIX_API_ASSIGNMENT and DEFINE_NIX_API_ASSIGNMENT_IMPL -> Assignment operators between NixPlatform types