update_engine: Disable puffin in minor version 4

This patch disables puffin for minor version 4, but keeps BROTLI_BSDIFF in the
same minor version.  Also adds puffin for minor version 5, but it does not
enable it.

BUG=b:72815313
BUG=chromium:810184
TEST=update_engine unittests pass
TEST=update_payload unittests pass
TEST=test_paycheck.sh on two images with minor version 4 pass
TEST=brill_update_payload generate/verify pass

Change-Id: I814fc126d67207501d419cff9c575cd13d877b48
Reviewed-on: https://chromium-review.googlesource.com/907223
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_metadata.proto b/update_metadata.proto
index ad377c7..7ff032a 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -169,8 +169,10 @@
     // On minor version 4 or newer, these operations are supported:
     ZERO = 6;  // Write zeros in the destination.
     DISCARD = 7;  // Discard the destination blocks, reading as undefined.
-    PUFFDIFF = 9;  // The data is in puffdiff format.
     BROTLI_BSDIFF = 10;  // Like SOURCE_BSDIFF, but compressed with brotli.
+
+    // On minor version 5 or newer, these operations are supported:
+    PUFFDIFF = 9;  // The data is in puffdiff format.
   }
   required Type type = 1;
   // The offset into the delta file (after the protobuf)