Fix buggy usage of global variables in the build system

build/module_args/mojo.gni

Index: build/module_args/mojo.gni
diff --git a/build/module_args/mojo.gni b/build/module_args/mojo.gni
index 51988d8c6d556bbd2773e434ff4a470ba76d5547..a20ef9e4ea12476db2584fd12cdf25cf4ba7938f 100644
--- a/build/module_args/mojo.gni
+++ b/build/module_args/mojo.gni
@@ -5,12 +5,12 @@
 # This variable should point to the parent directory of the Mojo SDK.
 mojo_sdk_root = "//"
 
-# To build the Mojo shell from source, set this variable to false. To use the
-# prebuilt shell, omit this variable or set it to true. Note that the prebuilt
+# To build the Mojo shell from source, set this variable to true. To use the
+# prebuilt shell, omit this variable or set it to false. Note that the prebuilt
 # shell will be used only on platforms for which it is published (currently
 # Linux and Android).
-use_prebuilt_mojo_shell = false
+build_mojo_shell_from_source = true
 
-# To build the network service from source, set this variable to false. To use
-# the prebuilt network service, omit this variable or set it to true.
-use_prebuilt_network_service = true
+# To build the network service from source, set this variable to true. To use
+# the prebuilt network service, omit this variable or set it to false.
+build_network_service_from_source = false