mojom: Error if multiple native types are mapped to the same mojom type.

mojo/public/tools/bindings/generate_type_mappings.py

Index: mojo/public/tools/bindings/generate_type_mappings.py
diff --git a/mojo/public/tools/bindings/generate_type_mappings.py b/mojo/public/tools/bindings/generate_type_mappings.py
index 71ab62f4fcf718a1f9485dc5c380a018d4b7547c..25dc49a190acaf6356c9279a368e1b66cc9cc57e 100755
--- a/mojo/public/tools/bindings/generate_type_mappings.py
+++ b/mojo/public/tools/bindings/generate_type_mappings.py
@@ -101,6 +101,10 @@ def ParseTypemap(typemap):
     pass_by_value = (match_result.group(3) and
                      match_result.group(3) == "pass_by_value")
 
+    assert mojom_type not in result, (
+        "Cannot map multiple native types (%s, %s) to the same mojom type: %s" %
+        (result[mojom_type]['typename'], native_type, mojom_type))
+
     result[mojom_type] = {
         'typename': native_type,
         'pass_by_value': pass_by_value,