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

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index de03e5577869d83b837a828f4cc9c49caff7014e..a2eb4daf1d95e46659ea349d3efa9a8778f29880 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -19,6 +19,7 @@
 #include "content/common/content_export.h"
 #include "content/common/content_param_traits.h"
 #include "content/common/content_security_policy_header.h"
+#include "content/common/download/mhtml_save_status.h"
 #include "content/common/frame_message_enums.h"
 #include "content/common/frame_owner_properties.h"
 #include "content/common/frame_replication_state.h"
@@ -1462,7 +1463,7 @@ IPC_MESSAGE_ROUTED2(FrameHostMsg_SerializedHtmlWithLocalLinksResponse,
 IPC_MESSAGE_ROUTED4(
     FrameHostMsg_SerializeAsMHTMLResponse,
     int /* job_id (used to match responses to requests) */,
-    bool /* true if success, false if error */,
+    content::MhtmlSaveStatus /* final success/failure status */,
     std::set<std::string> /* digests of uris of serialized resources */,
     base::TimeDelta /* how much time of the main render thread was used */)