ClientTagBasedModelTypeProcessor: Add ModelType to DCHECK output

We have reports of the DCHECK firing; this will let us figure out which
ModelType(s) hand inconsistent data to the processor.

Bug: 872360
Change-Id: I7d206a075870fe5be65234a6ebc5ba8d93e2099c
Reviewed-on: https://chromium-review.googlesource.com/c/1261396Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596646}

components/sync/model_impl/client_tag_based_model_type_processor.cc

@@ -133,7 +133,10 @@ void ClientTagBasedModelTypeProcessor::ModelReadyToSync(
     }
     model_type_state_ = batch->GetModelTypeState();
   } else {
-    DCHECK(commit_only_ || batch->TakeAllMetadata().empty());
+    // TODO(crbug.com/872360): This DCHECK can currently trigger if the user's
+    // persisted Sync metadata is in an inconsistent state.
+    DCHECK(commit_only_ || batch->TakeAllMetadata().empty())
+        << ModelTypeToString(type_);
     // First time syncing; initialize metadata.
     model_type_state_.mutable_progress_marker()->set_data_type_id(
         GetSpecificsFieldNumberFromModelType(type_));