[Sync::USS] Return the error in PasswordSyncBridge::MergeSyncData()

Bug: 902349
Change-Id: Ie6289a5fb4331e68f8da840de11fd53aca0e2c51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539978Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644466}

components/password_manager/core/browser/sync/password_sync_bridge.cc

@@ -440,7 +440,7 @@ base::Optional<syncer::ModelError> PasswordSyncBridge::MergeSyncData(
     password_store_sync_->NotifyLoginsChanged(password_store_changes);
   }
 
-  return base::nullopt;
+  return error;
 }
 
 base::Optional<syncer::ModelError> PasswordSyncBridge::ApplySyncChanges(

components/password_manager/core/browser/sync/password_sync_bridge_unittest.cc

@@ -651,6 +651,27 @@ TEST_F(PasswordSyncBridgeTest,
   EXPECT_TRUE(error);
 }
 
+// This tests that if storing model type state fails,
+// ShouldMergeSync() would return an error without crashing.
+TEST_F(
+    PasswordSyncBridgeTest,
+    ShouldMergeSyncRemoteAndLocalPasswordsWithErrorWhenStoreUpdateModelTypeStateFails) {
+  // Simulate failure in UpdateModelTypeState();
+  ON_CALL(*mock_sync_metadata_store_sync(), UpdateModelTypeState(_, _))
+      .WillByDefault(testing::Return(false));
+
+  sync_pb::ModelTypeState model_type_state;
+  model_type_state.set_initial_sync_done(true);
+
+  std::unique_ptr<syncer::MetadataChangeList> metadata_changes =
+      bridge()->CreateMetadataChangeList();
+  metadata_changes->UpdateModelTypeState(model_type_state);
+
+  base::Optional<syncer::ModelError> error =
+      bridge()->MergeSyncData(std::move(metadata_changes), {});
+  EXPECT_TRUE(error);
+}
+
 TEST_F(PasswordSyncBridgeTest, ShouldGetAllDataForDebuggingWithHiddenPassword) {
   const int kPrimaryKey1 = 1000;
   const int kPrimaryKey2 = 1001;