update_engine: refactor manifest parsing

DeltaPerformer write is quite long and complex. Let's move manifest
parsing to it's own method for readability.

Test: th
Change-Id: Id8ce50c3bd62959b1b70a4f05eef4718f3d30867
diff --git a/payload_consumer/delta_performer.h b/payload_consumer/delta_performer.h
index 1584dc6..76d64c5 100644
--- a/payload_consumer/delta_performer.h
+++ b/payload_consumer/delta_performer.h
@@ -246,6 +246,11 @@
   // to be able to perform a given install operation.
   bool CanPerformInstallOperation(const InstallOperation& operation);
 
+  bool ParseManifest(const char** c_bytes,
+                     size_t* count,
+                     ErrorCode* error,
+                     bool* should_return);
+
   // Process one InstallOperation
   bool ProcessOperation(const InstallOperation* op, ErrorCode* error);
   // Checks the integrity of the payload manifest. Returns true upon success,