[GTK][CMake] Add support for building the NetworkProcess

https://bugs.webkit.org/show_bug.cgi?id=127195

Reviewed by Daniel Bates.

.:

* Source/cmake/OptionsGTK.cmake: Turn on the network process and give it
a name like the WebProcess.

Source/WebKit2:

* CMakeLists.txt: Add shared network process build instructions.
* PlatformEfl.cmake: Move build instructions to the platform-independent file.
* PlatformGTK.cmake: Add some files to the source lists.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162536 268f45cc-cd09-0410-ab3c-d52691b4dbfc