[Password Manager] Fix fuzzing.

Bug: 1015234

Change-Id: Ibed37406b06a30f2d158d25c97994d256354c901
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865342
Auto-Submit: Vadym Doroshenko <dvadym@chromium.org>
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706833}

components/password_manager/core/browser/form_parsing/fuzzer/form_data_producer.cc

@@ -118,7 +118,8 @@ autofill::FormData GenerateWithDataAccessor(
   if (predictions) {
     predictions->driver_id = static_cast<int>(accessor->ConsumeNumber(32));
     predictions->form_signature =
-        static_cast<uint64_t>(accessor->ConsumeNumber(64));
+        (static_cast<uint64_t>(accessor->ConsumeNumber(32)) << 32) +
+        accessor->ConsumeNumber(32);
   }
 
   // And finally do the same for all the fields.