Add Assistant to grpcpp's visibility list

The Assistant service will use gRPC to get the SA bit from the
mothership API service. Also, the Assistant will have more usages of
gRPC calls while the Libassistant is transferring to V2 Design: see
go/libassistant-v2-api.

Bug: b/144870198
Test: Test with later changes
Change-Id: I5fa421b94c855cf6a69d6797e42a6464c30ff90d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055362Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743295}

third_party/grpc/BUILD.gn

@@ -39,6 +39,8 @@ static_library("grpcpp") {
   # chrome-security-reviews@ first before you decide to use gRPC.
   visibility = [
     "//chromecast/*",
+    "//chromeos/assistant/internal/*",
+    "//chromeos/services/assistant/*",
     "//remoting/*",
   ]
   sources = [