Reserve field 8 in DeepScanningClientRequest

This field is being used on the server side in cl/301451451.

Change-Id: I4fc7932a20b51354541886e42b833ad73344a4fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2110211
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Bettina Dea <bdea@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751795}

components/safe_browsing/core/proto/webprotect.proto

@@ -60,6 +60,9 @@ message DeepScanningClientRequest {
 
   // Sha256 digest of file.
   optional string digest = 7;
+
+  // Server-side only field.
+  reserved 8;
 }
 
 // Malware-specific response given back for scanned content.