Include IMGDIFF operation in minor version 4 or up.

Try imgdiff if both source and target contains gzip.

Test: unit test & generated a payload with boot.img
Bug: 26675118
Change-Id: I2861d9b953ffbdec44fdfb42cff5687698ea205e
diff --git a/payload_generator/extent_utils.h b/payload_generator/extent_utils.h
index 91729d5..3e45264 100644
--- a/payload_generator/extent_utils.h
+++ b/payload_generator/extent_utils.h
@@ -17,6 +17,7 @@
 #ifndef UPDATE_ENGINE_PAYLOAD_GENERATOR_EXTENT_UTILS_H_
 #define UPDATE_ENGINE_PAYLOAD_GENERATOR_EXTENT_UTILS_H_
 
+#include <string>
 #include <vector>
 
 #include "update_engine/payload_consumer/payload_constants.h"
@@ -76,6 +77,9 @@
 void ExtentsToVector(const google::protobuf::RepeatedPtrField<Extent>& extents,
                      std::vector<Extent>* out_vector);
 
+// Returns a string representing all extents in |extents|.
+std::string ExtentsToString(const std::vector<Extent>& extents);
+
 // Takes a pointer to extents |extents| and extents |extents_to_add|, and
 // merges them by adding |extents_to_add| to |extents| and normalizing.
 void ExtendExtents(