[Sync] Introduce ModelError for USS error handling.

components/sync/device_info/device_info_sync_bridge.cc

Index: components/sync/device_info/device_info_sync_bridge.cc
diff --git a/components/sync/device_info/device_info_sync_bridge.cc b/components/sync/device_info/device_info_sync_bridge.cc
index e1387abef361d7f0fcfff38b043161c3ef10810c..da0572377d8edf0112df5ff2b78efad40cd4189b 100644
--- a/components/sync/device_info/device_info_sync_bridge.cc
+++ b/components/sync/device_info/device_info_sync_bridge.cc
@@ -385,8 +385,9 @@ void DeviceInfoSyncBridge::OnReadAllMetadata(
 
 void DeviceInfoSyncBridge::OnCommit(Result result) {
   if (result != Result::SUCCESS) {
-    change_processor()->CreateAndUploadError(FROM_HERE,
-                                             "Failed a write to store.");
+    change_processor()->OnUnrecoverableError(
+        SyncError(FROM_HERE, SyncError::DATATYPE_ERROR,
+                  "Failed a write to store.", type()));
   }
 }
 
@@ -476,7 +477,8 @@ void DeviceInfoSyncBridge::ReportStartupErrorToSync(const std::string& msg) {
   // TODO(skym): Shouldn't need to log this here, reporting should always log.
   LOG(WARNING) << msg;
   change_processor()->OnMetadataLoaded(
-      change_processor()->CreateAndUploadError(FROM_HERE, msg), nullptr);
+      change_processor()->CreateAndUploadError(FROM_HERE, msg, type()),

[skym, 2017/01/05 17:03:49]

What if, we instead abandon the old error interfaces and write new ones. We
don't have to NOTREACHED in Copy(). We don't have to supply a type to
CreateAndUploadError(). I don't really like the existence of
CreateAndUploadError(). First, it shouldn't upload. that should be done in
OnUnrecoverableError() or OnMetadataLoaded(). Except AndUpload is in the name.

But, I don't think CreateAndUploadError() is even needed if we were to remove
SyncErrors from OnMetadataLoaded(). Can we instead have the bridge just call
OnUnrecoverableError() and never get around to OnMetadataLoaded? Making sure
OnMetadataLoaded() is always called is a hassle for the bridge, and can be error
prone when encountering errors.

Hmm, you don't even need to start with a new interface. Just create a one off
method in the Proc.

void SharedModelTypeProcessor::ReportUnrecoverableError(const
tracked_objects::Location& location, const std::string& message) {
  ...
}

[maxbogue, 2017/01/07 00:12:39]

Done!

+      nullptr);
 }
 
 }  // namespace syncer