Define mojo_build_mojo_shell_from_source = true

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 21d9dbd6dcdc5d24501fbbcdbf1d3d2626850784..efa553560bdd3d00d2f95ae717d5287952b3d679 100644
--- a/build/module_args/mojo.gni
+++ b/build/module_args/mojo.gni
@@ -17,6 +17,9 @@ mojo_disable_dart_apptest_framework = true
 mojo_network_service_root = "//mojo/services"
 
 declare_args() {
+  # The mojo shell is forked and built in the chromium repo.
+  mojo_build_mojo_shell_from_source = true
+
   # Specify prebuilt network service mojo file location rather than download
   # from gs://mojo.
   #