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

content/common/content_param_traits_macros.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Singly or Multiply-included shared traits file depending on circumstances.
 // This allows the use of IPC serialization macros in more than one IPC message
 // file.
 #ifndef CONTENT_COMMON_CONTENT_PARAM_TRAITS_MACROS_H_
 #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"
 #include "content/public/common/resource_type.h"
 #include "ipc/ipc_message_macros.h"
 #include "third_party/WebKit/public/platform/WebAddressSpace.h"
 #include "third_party/WebKit/public/platform/WebInputEvent.h"
 #include "third_party/WebKit/public/platform/WebPageVisibilityState.h"
 #include "third_party/WebKit/public/web/WebCompositionUnderline.h"
 #include "third_party/WebKit/public/web/WebContentSecurityPolicy.h"
 #include "third_party/WebKit/public/web/WebSharedWorkerCreationContextType.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 
 #undef IPC_MESSAGE_EXPORT
 #define IPC_MESSAGE_EXPORT CONTENT_EXPORT
 
 IPC_ENUM_TRAITS_MAX_VALUE(content::InputEventAckState,
                           content::INPUT_EVENT_ACK_STATE_MAX)
 IPC_ENUM_TRAITS_MAX_VALUE(content::ResourceType,
                           content::RESOURCE_TYPE_LAST_TYPE - 1)
 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,
                           blink::WebContentSecurityPolicyTypeLast)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebAddressSpace, blink::WebAddressSpaceLast)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebSharedWorkerCreationContextType,
                           blink::WebSharedWorkerCreationContextTypeLast)
 IPC_ENUM_TRAITS_MIN_MAX_VALUE(blink::WebInputEvent::Type,
                               blink::WebInputEvent::TypeFirst,
                               blink::WebInputEvent::TypeLast)
 IPC_ENUM_TRAITS_MAX_VALUE(blink::WebPageVisibilityState,
                           blink::WebPageVisibilityStateLast)
 
 IPC_STRUCT_TRAITS_BEGIN(blink::WebCompositionUnderline)
   IPC_STRUCT_TRAITS_MEMBER(startOffset)
   IPC_STRUCT_TRAITS_MEMBER(endOffset)
   IPC_STRUCT_TRAITS_MEMBER(color)
   IPC_STRUCT_TRAITS_MEMBER(thick)
   IPC_STRUCT_TRAITS_MEMBER(backgroundColor)
 IPC_STRUCT_TRAITS_END()
 
 #endif  // CONTENT_COMMON_CONTENT_PARAM_TRAITS_MACROS_H_