grpc: fix TSAN race introduced in #3545. (#3635)
grpc: fix TSAN race introduced in #3545. (#3635)

This showed up in some internal tests.

Signed-off-by: Harvey Tuch <c185081708e02336640916420d0c772668570de5@google.com>
dnoe/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,dnoe/envoy,jrajahalme/envoy,istio/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,eklitzke/envoy,eklitzke/envoy,eklitzke/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,lizan/envoy,dnoe/envoy,envoyproxy/envoy,lyft/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy,istio/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,dnoe/envoy,lyft/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,jrajahalme/envoy,lyft/envoy,jrajahalme/envoy,lizan/envoy,eklitzke/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy-wasm,istio/envoy,dnoe/envoy,lizan/envoy,lyft/envoy,envoyproxy/envoy