Fix buggy usage of global variables in the build system

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
index 23aac005706ae2779f9303419b7555a6fa8f0397..bc90a236f9b52dbe39fbcb69f0c38a8b9560f9ee 100644
--- a/mojo/public/mojo.gni
+++ b/mojo/public/mojo.gni
@@ -6,13 +6,16 @@ import("//build/module_args/mojo.gni")
 
 # If using the prebuilt shell, gate its usage by the platforms for which it is
 # published.
-if (!defined(use_prebuilt_mojo_shell) || use_prebuilt_mojo_shell) {
+use_prebuilt_mojo_shell = false
+if (!defined(build_mojo_shell_from_source) || !build_mojo_shell_from_source) {
   use_prebuilt_mojo_shell = is_linux || is_android
 }
 
 # If using the prebuilt network service, gate its usage by the platforms for
 # which it is published.
-if (!defined(use_prebuilt_network_service) || use_prebuilt_network_service) {
+use_prebuilt_network_service = false
+if (!defined(build_network_service_from_source) ||
+    !build_network_service_from_source) {
   use_prebuilt_network_service = is_linux || is_android
 }