[Impeller] Fail GN when building an enabled Impeller backend is impossible.

impeller/renderer/backend/BUILD.gn

@@ -8,14 +8,17 @@ group("backend") {
   public_deps = []
 
   if (impeller_enable_metal) {
+    assert(is_mac || is_ios)
     public_deps += [ "metal" ]
   }
 
   if (impeller_enable_opengles) {
+    assert(is_mac || is_linux || is_win || is_android)
     public_deps += [ "gles" ]
   }
 
   if (impeller_enable_vulkan) {
+    assert(is_mac || is_linux || is_win || is_android)
     public_deps += [ "vulkan" ]
   }
 }