Forward bundle product_type to the compile_xcassets tool.

tools/gn/substitution_type.h

Index: tools/gn/substitution_type.h
diff --git a/tools/gn/substitution_type.h b/tools/gn/substitution_type.h
index e82ecd61073c72dcc8b276f2384156dd110aada4..84524fb9e40c270128ccb3676b74c688cd1909e4 100644
--- a/tools/gn/substitution_type.h
+++ b/tools/gn/substitution_type.h
@@ -69,6 +69,9 @@ enum SubstitutionType {
   SUBSTITUTION_BUNDLE_EXECUTABLE_DIR,  // {{bundle_executable_dir}}
   SUBSTITUTION_BUNDLE_PLUGINS_DIR,  // {{bundle_plugins_dir}}
 
+  // Valid for compile_xcassets tool.
+  SUBSTITUTION_BUNDLE_PRODUCT_TYPE,  // {{bundle_product_type}}
+
   // Used only for the args of actions.
   SUBSTITUTION_RSP_FILE_NAME,  // {{response_file_name}}