BootControl: is slot marked successful

Test: update_engine_unittests
Bug: 147696014

Change-Id: I316efda7a0936e2dee0f766efb5c18ff30790274

common/boot_control_interface.h

diff --git a/common/boot_control_interface.h b/common/boot_control_interface.h
index 41fe4ea..3906e2f 100644
--- a/common/boot_control_interface.h
+++ b/common/boot_control_interface.h
@@ -85,6 +85,9 @@
   // of the operation.
   virtual bool MarkBootSuccessfulAsync(base::Callback<void(bool)> callback) = 0;
 
+  // Check if |slot| is marked boot successfully.
+  virtual bool IsSlotMarkedSuccessful(Slot slot) const = 0;
+
   // Return the dynamic partition control interface.
   virtual DynamicPartitionControlInterface* GetDynamicPartitionControl() = 0;