Define MOJO_SHELL_CLIENT uniformly across content

content/public/common/BUILD.gn

Index: content/public/common/BUILD.gn
diff --git a/content/public/common/BUILD.gn b/content/public/common/BUILD.gn
index 63f5fa9fff551becc2c58365799a039a5d6d9e6a..9c1e0f771d49464b6f302489b4f81735726a644a 100644
--- a/content/public/common/BUILD.gn
+++ b/content/public/common/BUILD.gn
@@ -44,6 +44,15 @@ config("static_switches_defines") {
   defines = [ "COMPILE_CONTENT_STATICALLY" ]
 }
 
+# Set in GN builds, triggering behavior in content when run from an external
+# Mojo shell.
+config("mojo_shell_client") {
+  # This configuration has only been tested on these platforms.
+  if (is_win || is_linux || is_chromeos) {
+    defines = [ "MOJO_SHELL_CLIENT" ]
+  }
+}
+
 # This target allows you to use the content_switches constants and statically
 # link to it, without depending on the rest of content. This is only for use
 # without content, or you will get multiply defined symbols.