protobuf: Add generator_visibility flag to proto_library.gni

This is to make code generating action visible to some additional
targets so that targets using generated files can depend on code
generating action directly. By doing this, we can change public_deps to
deps for code generating action in proto compile target to remove
unnecessary transitive inputs dependency (e.g. .o file -> .proto files
deps).

See https://crrev.com/c/6414089 about how to use the added arg.

Bug: 402356473
Change-Id: Ie95a4cecf6d4e03d5320e2aa3f7a5f7e7a3c903a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6414530
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440096}