Make Dawn "gn check" by default (except SPIRV-Tools)

This required adding some missing dependencies, splitting public headers
of libdawn_[native|wire] so they aren't hidden in the
libdawn_[native|wire]_sources targets, and making unittests depend on
sources directly instead of static libraries (which is almost equivalent).

As a byproduct, Empty.cpp is no longer needed and is removed.

third_party/BUILD.gn

@@ -283,7 +283,6 @@ static_library("glfw") {
     "${glfw_dir}/include/GLFW/glfw3.h",
     "${glfw_dir}/include/GLFW/glfw3native.h",
     "${glfw_dir}/src/context.c",
-    "${glfw_dir}/src/glfw_config.h",
     "${glfw_dir}/src/init.c",
     "${glfw_dir}/src/input.c",
     "${glfw_dir}/src/internal.h",