Handle resume of VABC updates by emitting labels

To support resuming an update with Virtual AB Compression, we emit
labels in between operations. After writing all SOURCE_COPY, we
emit label 0. Each time we finished writing an InstallOp, we emit
a label incremented by 1. When resuming, we pass the label to CowWriter.

Test: treehugger
     1. update_device.py ota.zip
     --extra-headers="SWITCH_SLOT_ON_REBOOT=0"
     2. update_device.py ota.zip
     3. Verify that 2 did not re-start the entire update,
        only fs verification and postinstall may re-run.
Change-Id: I785cd04a35457181621ed7b8c0be9a46b6004b7b
diff --git a/payload_consumer/partition_writer_unittest.cc b/payload_consumer/partition_writer_unittest.cc
index 1ef4783..91e5e26 100644
--- a/payload_consumer/partition_writer_unittest.cc
+++ b/payload_consumer/partition_writer_unittest.cc
@@ -96,8 +96,9 @@
     install_part_.target_size = blob_data.size();
 
     ErrorCode error;
-    EXPECT_TRUE(writer_.Init(&install_plan_, true));
+    EXPECT_TRUE(writer_.Init(&install_plan_, true, 0));
     EXPECT_TRUE(writer_.PerformSourceCopyOperation(op, &error));
+    writer_.CheckpointUpdateProgress(1);
 
     brillo::Blob output_data;
     EXPECT_TRUE(utils::ReadFile(target_partition.path(), &output_data));