[Sync] Remove ModelError::IsSet() in favor of base::Optional.

components/reading_list/ios/reading_list_store_unittest.mm

Index: components/reading_list/ios/reading_list_store_unittest.mm
diff --git a/components/reading_list/ios/reading_list_store_unittest.mm b/components/reading_list/ios/reading_list_store_unittest.mm
index d25ce02ce05af8d11496422d05ec281e53a4b8a1..b13199e4f2354708c76ae655de95b6a9baa1b859 100644
--- a/components/reading_list/ios/reading_list_store_unittest.mm
+++ b/components/reading_list/ios/reading_list_store_unittest.mm
@@ -220,8 +220,8 @@ TEST_F(ReadingListStoreTest, SyncMergeOneEntry) {
 
   std::unique_ptr<syncer::MetadataChangeList> metadata_changes(
       reading_list_store_->CreateMetadataChangeList());
-  const syncer::ModelError error = reading_list_store_->MergeSyncData(
-      std::move(metadata_changes), remote_input);
+  auto error = reading_list_store_->MergeSyncData(std::move(metadata_changes),

[skym, 2017/01/10 16:24:41]

Why are these error variables being stored? They're never used. Should they be
checked?

[maxbogue, 2017/01/10 17:53:50]

I don't know; I'll ask Olivier about it.

+                                                  remote_input);
   AssertCounts(0, 0, 1, 0, 0);
   EXPECT_EQ(sync_added_.size(), 1u);
   EXPECT_EQ(sync_added_.count("http://read.example.com/"), 1u);
@@ -242,7 +242,7 @@ TEST_F(ReadingListStoreTest, ApplySyncChangesOneAdd) {
 
   add_changes.push_back(syncer::EntityChange::CreateAdd(
       "http://read.example.com/", data.PassToPtr()));
-  syncer::ModelError error = reading_list_store_->ApplySyncChanges(
+  auto error = reading_list_store_->ApplySyncChanges(
       reading_list_store_->CreateMetadataChangeList(), add_changes);
   AssertCounts(0, 0, 1, 0, 0);
   EXPECT_EQ(sync_added_.size(), 1u);
@@ -268,7 +268,7 @@ TEST_F(ReadingListStoreTest, ApplySyncChangesOneMerge) {
   syncer::EntityChangeList add_changes;
   add_changes.push_back(syncer::EntityChange::CreateAdd(
       "http://unread.example.com/", data.PassToPtr()));
-  syncer::ModelError error = reading_list_store_->ApplySyncChanges(
+  auto error = reading_list_store_->ApplySyncChanges(
       reading_list_store_->CreateMetadataChangeList(), add_changes);
   AssertCounts(1, 0, 0, 0, 1);
   EXPECT_EQ(sync_merged_.size(), 1u);
@@ -297,7 +297,7 @@ TEST_F(ReadingListStoreTest, ApplySyncChangesOneIgnored) {
   syncer::EntityChangeList add_changes;
   add_changes.push_back(syncer::EntityChange::CreateAdd(
       "http://unread.example.com/", data.PassToPtr()));
-  syncer::ModelError error = reading_list_store_->ApplySyncChanges(
+  auto error = reading_list_store_->ApplySyncChanges(
       reading_list_store_->CreateMetadataChangeList(), add_changes);
   AssertCounts(1, 0, 0, 0, 1);
   EXPECT_EQ(sync_merged_.size(), 1u);
@@ -307,7 +307,7 @@ TEST_F(ReadingListStoreTest, ApplySyncChangesOneRemove) {
   syncer::EntityChangeList delete_changes;
   delete_changes.push_back(
       syncer::EntityChange::CreateDelete("http://read.example.com/"));
-  syncer::ModelError error = reading_list_store_->ApplySyncChanges(
+  auto error = reading_list_store_->ApplySyncChanges(
       reading_list_store_->CreateMetadataChangeList(), delete_changes);
   AssertCounts(0, 0, 0, 1, 0);
   EXPECT_EQ(sync_removed_.size(), 1u);