Use UpdateUsesSnapshotCompression to determine if VABC is used

Test: th
Change-Id: Ia04f92d46da34fcd28d7e97c24b6e02fd676e1c7
diff --git a/common/dynamic_partition_control_interface.h b/common/dynamic_partition_control_interface.h
index 61127d4..8b29c3b 100644
--- a/common/dynamic_partition_control_interface.h
+++ b/common/dynamic_partition_control_interface.h
@@ -69,6 +69,9 @@
   // Return the feature flags of Virtual A/B on this device.
   virtual FeatureFlag GetVirtualAbFeatureFlag() = 0;
   // Return the feature flags of Virtual A/B Compression on this device.
+  // This function will tell you if current device supports VABC. However, it
+  // DOES NOT tell you if VABC is used for current OTA update. For that, use
+  // UpdateUsesSnapshotCompression.
   virtual FeatureFlag GetVirtualAbCompressionFeatureFlag() = 0;
 
   // Attempt to optimize |operation|.