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)

[nasko, 2016/11/23 18:07:34]

Why the need for static casting? Can't we just define the enum with LAST member
like the rest and follow the common pattern?

[carlosk, 2016/11/23 20:08:40]

Static casting is needed for the subtraction operation. I considered this other
pattern (also used above for ResourceType) somewhat better as it makes adding
new values to the enum simpler. The *LAST pattern requires that new values added
as a new value and also to the *LAST line to set its value, whereas this one
requires only the former step. I don't have strong feelings either way so WDYT?

[nasko, 2016/11/24 00:07:31]

I prefer the compiler do the work for us, which means I'd rather not have static
casts. Devs should be capable enough to fix the LAST value when adding a new one
to the enum.

[carlosk, 2016/11/24 00:21:11]

Done.

 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebContentSecurityPolicySource,
                           blink::WebContentSecurityPolicySourceLast)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebContentSecurityPolicyType,