Fix DCHECK in AuditorResult

While testing the related bug on a debug version of the auditor, I found
that a DCHECK is missing a case. It is possible to have an AuditorResult
with an empty message of type ERROR_MUTABLE_TAG. An AnnotationInstance
will specifically create an AuditorResult of type ERROR_MUTABLE_TAG
without a message. The detailed text provided by the AuditorResult
itself should give sufficient context.

Bug: 1005537
Change-Id: I39d66e758ed5b847430a7385e1d15bd33470d0ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849990Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Nicolas Arciniega <niarci@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#704720}

tools/traffic_annotation/auditor/auditor_result.cc

@@ -31,7 +31,8 @@ AuditorResult::AuditorResult(Type type,
          type == AuditorResult::Type::ERROR_MISSING_SECOND_ID ||
          type == AuditorResult::Type::ERROR_DIRECT_ASSIGNMENT ||
          type == AuditorResult::Type::ERROR_TEST_ANNOTATION ||
-         type == AuditorResult::Type::ERROR_INVALID_OS);
+         type == AuditorResult::Type::ERROR_INVALID_OS ||
+         type == AuditorResult::Type::ERROR_MUTABLE_TAG);
   if (!message.empty())
     details_.push_back(message);
 }