Fail when saving page as MHTML provides information about the cause.

content/common/content_param_traits_macros.h

Index: content/common/content_param_traits_macros.h
diff --git a/content/common/content_param_traits_macros.h b/content/common/content_param_traits_macros.h
index 40767e927632badbe2eb341a3c05becacac4ab2e..27d6778c1feb88b116eec8783f5ed469ea2e64c5 100644
--- a/content/common/content_param_traits_macros.h
+++ b/content/common/content_param_traits_macros.h
@@ -9,6 +9,7 @@
 #define CONTENT_COMMON_CONTENT_PARAM_TRAITS_MACROS_H_
 
 #include "content/common/content_export.h"
+#include "content/common/download/mhtml_save_status.h"
 #include "content/common/input/input_event_ack_state.h"
 #include "content/public/common/request_context_frame_type.h"
 #include "content/public/common/request_context_type.h"
@@ -33,6 +34,9 @@ IPC_ENUM_TRAITS_MAX_VALUE(content::RequestContextType,
                           content::REQUEST_CONTEXT_TYPE_LAST)
 IPC_ENUM_TRAITS_MAX_VALUE(content::RequestContextFrameType,
                           content::REQUEST_CONTEXT_FRAME_TYPE_LAST)
+IPC_ENUM_TRAITS_MAX_VALUE(
+    content::MhtmlSaveStatus,
+    static_cast<int>(content::MhtmlSaveStatus::STATUS_COUNT) - 1)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebContentSecurityPolicySource,
                           blink::WebContentSecurityPolicySourceLast)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebContentSecurityPolicyType,