Update google-protobuf and remove workaround

In order to comply with the no-unsafe-eval CSP, we worked around issue
https://github.com/protocolbuffers/protobuf/issues/6770 by including a
modified version of google-protobuf under the src/third_party folder.

This CL updates the google-protobuf version to one which fixes this bug,
and removes the workaround.

Change-Id: Ida7943bad452ee930defbc136602a34910a41977
diff --git a/package.json b/package.json
index 3d4fa5f..3752313 100644
--- a/package.json
+++ b/package.json
@@ -45,7 +45,7 @@
     "@material/menu": "^10.0.0",
     "@material/tooltip": "^12.0.0",
     "async-mutex": "^0.3.2",
-    "google-protobuf": "^3.18.0-rc.2",
+    "google-protobuf": "^3.19.3",
     "grpc-web": "^1.2.1",
     "idb": "^6.1.2",
     "poll-until-promise": "^3.6.1",