update_payload: Allow check for given metadata size

Allow passing metadata size to check_update_payload so we can verify the
metadata size in omaha equals to the one in the payload.

BUG=chromium:820243
TEST=run paycheck.py with both valid and invalid metadata sizes reports as expected
TEST=unittests

Change-Id: Ib41ce77af77636fffec6752201c363e7fbbf868d
Reviewed-on: https://chromium-review.googlesource.com/955679
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/scripts/update_payload/payload.py b/scripts/update_payload/payload.py
index 380d6d0..15f66d0 100644
--- a/scripts/update_payload/payload.py
+++ b/scripts/update_payload/payload.py
@@ -273,14 +273,15 @@
     return not self.IsDelta()
 
   def Check(self, pubkey_file_name=None, metadata_sig_file=None,
-            report_out_file=None, assert_type=None, block_size=0,
-            rootfs_part_size=0, kernel_part_size=0, allow_unhashed=False,
-            disabled_tests=()):
+            metadata_size=0, report_out_file=None, assert_type=None,
+            block_size=0, rootfs_part_size=0, kernel_part_size=0,
+            allow_unhashed=False, disabled_tests=()):
     """Checks the payload integrity.
 
     Args:
       pubkey_file_name: public key used for signature verification
       metadata_sig_file: metadata signature, if verification is desired
+      metadata_size: metadata size, if verification is desired
       report_out_file: file object to dump the report to
       assert_type: assert that payload is either 'full' or 'delta'
       block_size: expected filesystem / payload block size
@@ -300,6 +301,7 @@
         allow_unhashed=allow_unhashed, disabled_tests=disabled_tests)
     helper.Run(pubkey_file_name=pubkey_file_name,
                metadata_sig_file=metadata_sig_file,
+               metadata_size=metadata_size,
                rootfs_part_size=rootfs_part_size,
                kernel_part_size=kernel_part_size,
                report_out_file=report_out_file)