Revert "Revert "AU: do not copy filesystem during full updates""

This reverts commit d1cd325c3135d88498483da811b594ba6b91ce42

The problem that caused all autotests to fail with the original CL has now been rectified; lab devservers were updated to send the correct delta flag in their omaha response.

Change-Id: I664afb33f72856572baaa658cbd473c07271af36
Reviewed-on: https://gerrit.chromium.org/gerrit/56600
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/delta_performer.h b/delta_performer.h
index 1f8f5cb..33e39d7 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -193,6 +193,10 @@
   bool CanPerformInstallOperation(
       const DeltaArchiveManifest_InstallOperation& operation);
 
+  // Checks the integrity of the payload manifest. Returns true upon success,
+  // false otherwise.
+  ErrorCode ValidateManifest();
+
   // Validates that the hash of the blobs corresponding to the given |operation|
   // matches what's specified in the manifest in the payload.
   // Returns kErrorCodeSuccess on match or a suitable error code otherwise.