Add IsDynamicPartition API to easily check if a partition is dynamic

Test: treehugger
Change-Id: I655e4d6114a25c58857679985cd5e08581c0faab
diff --git a/common/dynamic_partition_control_stub.h b/common/dynamic_partition_control_stub.h
index 94540f4..0f428ab 100644
--- a/common/dynamic_partition_control_stub.h
+++ b/common/dynamic_partition_control_stub.h
@@ -68,6 +68,8 @@
 
   bool MapAllPartitions() override;
   bool UnmapAllPartitions() override;
+
+  bool IsDynamicPartition(const std::string& part_name) override;
 };
 }  // namespace chromeos_update_engine