[gRPC] Add a note for using gRPC outside remoting

Per discussion in crrev.com/c/2055362, it's generally not safe to use
gRPC outside Chrome Remote Desktop, especially in Chrome. This CL adds
a note to the BUILD.gn file to encourage people to discuss this with
Chrome security team before they decide to use gRPC.

Change-Id: I9218059a977d5aea785cd70621f7ab2c428ecc0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062920
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742363}

third_party/grpc/BUILD.gn

@@ -34,6 +34,9 @@ config("nanopb_config") {
 # want to use cc_grpc_library() from grpc_library.gni directly.
 static_library("grpcpp") {
   # This is currently only used by remoting and chromecast.
+  # NOTE: Using gRPC outside remoting may have security concerns and other
+  # complications when building the binary. Please contact
+  # chrome-security-reviews@ first before you decide to use gRPC.
   visibility = [
     "//chromecast/*",
     "//remoting/*",