Firestore: protoc-generated serialization update.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/firestore/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:5a96c2c5c6f9570cc9556b63dc9ce1838777fd9166b5b64e43ad8e0ecee2fe2c
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/firestore/artman_firestore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/firestore-v1beta1.
synthtool > Replaced 'from google.cloud import firestore_v1beta1' in tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py.
synthtool > Replaced 'client = firestore_v1beta1.FirestoreClient' in tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/firestore/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/gapic/firestore_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/gapic/transports/firestore_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/common_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/document_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/common_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/gapic/firestore_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/firestore_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/query_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/document_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/query_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/write_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/write_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/firestore/google/cloud/firestore_v1beta1/proto/firestore_pb2.py
All done! ✨ 🍰 ✨
15 files reformatted, 43 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[tseaver]

Systest failure reported in #7105.

[tseaver]

No .proto file changes.