[Nearby] Clear last_x_metadata in UnregisterShareTarget

The last_incoming_metadata/last_outgoing_metadata was not ever getting
cleared on a successful transfer. This would cause a share target to
show up in discovery right away, but it would be a dead target that
would fail if you tried to use it.

Change-Id: I9679a6200fd948d0016484fee645c24db6f2ad77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425745
Auto-Submit: James Vecore <vecore@google.com>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809738}

chrome/browser/nearby_sharing/nearby_sharing_service_impl.cc

@@ -3179,10 +3179,18 @@ void NearbySharingServiceImpl::UnregisterShareTarget(
                   << share_target.device_name;
 
   if (share_target.is_incoming) {
+    if (last_incoming_metadata_ &&
+        last_incoming_metadata_->first.id == share_target.id) {
+      last_incoming_metadata_.reset();
+    }
     incoming_share_target_info_map_.erase(share_target.id);
     // Clear legacy incoming payloads to release resource
     nearby_connections_manager_->ClearIncomingPayloads();
   } else {
+    if (last_outgoing_metadata_ &&
+        last_outgoing_metadata_->first.id == share_target.id) {
+      last_outgoing_metadata_.reset();
+    }
     // Find the endpoint id that matches the given share target.
     base::Optional<std::string> endpoint_id;
     auto it = outgoing_share_target_info_map_.find(share_target.id);