Support verity writes in VABC

Test: generate an OTA with verity enabled, install it
Bug: 173432386

Change-Id: I14cccb1bb339c9824a95b8e42ac5144cb8b75c3f
diff --git a/payload_consumer/filesystem_verifier_action.h b/payload_consumer/filesystem_verifier_action.h
index 1f527c9..78634cb 100644
--- a/payload_consumer/filesystem_verifier_action.h
+++ b/payload_consumer/filesystem_verifier_action.h
@@ -126,14 +126,8 @@
   size_t partition_index_{0};
 
   // If not null, the FileDescriptor used to read from the device.
-  // |read_fd_| and |write_fd_| will be initialized when we begin hashing a
-  // partition. They will be deallocated once we encounter an error or
-  // successfully finished hashing.
-  FileDescriptorPtr read_fd_;
-  // If not null, the FileDescriptor used to write to the device.
-  // For VABC, this will be different from |read_fd_|. For other cases
-  // this can be the same as |read_fd_|.
-  FileDescriptorPtr write_fd_;
+  // verity writer might attempt to write to this fd, if verity is enabled.
+  FileDescriptorPtr partition_fd_;
 
   // Buffer for storing data we read.
   brillo::Blob buffer_;