mojom: Add associative arrays to the mojom language.

mojo/public/cpp/bindings/lib/validation_errors.h

Index: mojo/public/cpp/bindings/lib/validation_errors.h
diff --git a/mojo/public/cpp/bindings/lib/validation_errors.h b/mojo/public/cpp/bindings/lib/validation_errors.h
index ef5ab9e9093f77fea0ae5abe532bd967c066fd5c..2abb1b7545620b73f55d6f6aa10e6ea19f74a3cf 100644
--- a/mojo/public/cpp/bindings/lib/validation_errors.h
+++ b/mojo/public/cpp/bindings/lib/validation_errors.h
@@ -44,6 +44,9 @@ enum ValidationError {
   // |flags| in the message header indicates that a request ID is required but
   // there isn't one.
   VALIDATION_ERROR_MESSAGE_HEADER_MISSING_REQUEST_ID,
+  // Two parallel arrays which are supposed to represent a map have different
+  // lengths.
+  VALIDATION_ERROR_DIFFERENT_SIZED_ARRAYS_IN_MAP
 };
 
 const char* ValidationErrorToString(ValidationError error);