update_engine: Place DlcserviceInterface factory in interface header

Cleanup file separation for DlcserviceInterface factory as a completely
separate header is not necesary.

BUG=none
TEST=FEATURES=test emerge-$B update_engine
TEST=USE="${USE} -dlc" FEATURES=test emerge-$B update_engine

Change-Id: I6c9cfcac1b7d0079e9b0cc4d4e984fafc5f1923f
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2202617
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Andrew Lassalle <andrewlassalle@chromium.org>
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/common/dlcservice_interface.h b/common/dlcservice_interface.h
index 70b74ab..7b57710 100644
--- a/common/dlcservice_interface.h
+++ b/common/dlcservice_interface.h
@@ -17,6 +17,7 @@
 #ifndef UPDATE_ENGINE_COMMON_DLCSERVICE_INTERFACE_H_
 #define UPDATE_ENGINE_COMMON_DLCSERVICE_INTERFACE_H_
 
+#include <memory>
 #include <string>
 #include <vector>
 
@@ -49,6 +50,10 @@
   DISALLOW_COPY_AND_ASSIGN(DlcServiceInterface);
 };
 
+// This factory function creates a new DlcServiceInterface instance for the
+// current platform.
+std::unique_ptr<DlcServiceInterface> CreateDlcService();
+
 }  // namespace chromeos_update_engine
 
 #endif  // UPDATE_ENGINE_COMMON_DLCSERVICE_INTERFACE_H_